diff --git a/src/ecmascript/spidermonkey/document.c b/src/ecmascript/spidermonkey/document.c
index 13e647bfd..79bc700f6 100644
--- a/src/ecmascript/spidermonkey/document.c
+++ b/src/ecmascript/spidermonkey/document.c
@@ -164,7 +164,12 @@ document_get_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 		astring_to_jsval(ctx, vp, get_uri_string(document->uri, URI_ORIGINAL));
 		break;
 	default:
-		INTERNAL("Invalid ID %d in document_get_property().", JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case
+		 * and leave *@vp unchanged.  Do the same here.
+		 * (Actually not quite the same, as we already used
+		 * @undef_to_jsval.)  */
 		break;
 	}
 
diff --git a/src/ecmascript/spidermonkey/form.c b/src/ecmascript/spidermonkey/form.c
index 430ea2c29..659e2043d 100644
--- a/src/ecmascript/spidermonkey/form.c
+++ b/src/ecmascript/spidermonkey/form.c
@@ -262,7 +262,12 @@ input_get_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 		if (fc->type == FC_SELECT) int_to_jsval(ctx, vp, fs->state);
 		break;
 	default:
-		INTERNAL("Invalid ID %d in input_get_property().", JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case
+		 * and leave *@vp unchanged.  Do the same here.
+		 * (Actually not quite the same, as we already used
+		 * @undef_to_jsval.)  */
 		break;
 	}
 
@@ -374,7 +379,10 @@ input_set_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 		break;
 
 	default:
-		INTERNAL("Invalid ID %d in input_set_property().", JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case.
+		 * Do the same here.  */
 		return JS_TRUE;
 	}
 
@@ -904,7 +912,12 @@ form_get_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 		break;
 
 	default:
-		INTERNAL("Invalid ID %d in form_get_property().", JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case
+		 * and leave *@vp unchanged.  Do the same here.
+		 * (Actually not quite the same, as we already used
+		 * @undef_to_jsval.)  */
 		break;
 	}
 
@@ -985,7 +998,10 @@ form_set_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 		break;
 
 	default:
-		INTERNAL("Invalid ID %d in form_set_property().", JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case.
+		 * Do the same here.  */
 		break;
 	}
 
diff --git a/src/ecmascript/spidermonkey/location.c b/src/ecmascript/spidermonkey/location.c
index 161a43ae8..49cb74da7 100644
--- a/src/ecmascript/spidermonkey/location.c
+++ b/src/ecmascript/spidermonkey/location.c
@@ -170,7 +170,12 @@ location_get_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 		astring_to_jsval(ctx, vp, get_uri_string(vs->uri, URI_ORIGINAL));
 		break;
 	default:
-		INTERNAL("Invalid ID %d in location_get_property().", JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case
+		 * and leave *@vp unchanged.  Do the same here.
+		 * (Actually not quite the same, as we already used
+		 * @undef_to_jsval.)  */
 		break;
 	}
 
diff --git a/src/ecmascript/spidermonkey/navigator.c b/src/ecmascript/spidermonkey/navigator.c
index aa0629a95..6c7ceab10 100644
--- a/src/ecmascript/spidermonkey/navigator.c
+++ b/src/ecmascript/spidermonkey/navigator.c
@@ -134,7 +134,12 @@ navigator_get_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 	}
 		break;
 	default:
-		INTERNAL("Invalid ID %d in navigator_get_property().", JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case
+		 * and leave *@vp unchanged.  Do the same here.
+		 * (Actually not quite the same, as we already used
+		 * @undef_to_jsval.)  */
 		break;
 	}
 
diff --git a/src/ecmascript/spidermonkey/unibar.c b/src/ecmascript/spidermonkey/unibar.c
index 14d2570a0..30ddf44cf 100644
--- a/src/ecmascript/spidermonkey/unibar.c
+++ b/src/ecmascript/spidermonkey/unibar.c
@@ -120,7 +120,10 @@ unibar_get_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 #undef unibar_fetch
 		break;
 	default:
-		INTERNAL("Invalid ID %d in unibar_get_property().", JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case
+		 * and leave *@vp unchanged.  Do the same here.  */
 		break;
 	}
 
@@ -170,7 +173,10 @@ unibar_set_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 		register_bottom_half(update_status, NULL);
 		break;
 	default:
-		INTERNAL("Invalid ID %d in unibar_set_property().", JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case.
+		 * Do the same here.  */
 		return JS_TRUE;
 	}
 
diff --git a/src/ecmascript/spidermonkey/window.c b/src/ecmascript/spidermonkey/window.c
index 8872bc667..d0f680d58 100644
--- a/src/ecmascript/spidermonkey/window.c
+++ b/src/ecmascript/spidermonkey/window.c
@@ -234,7 +234,12 @@ found_parent:
 		break;
 	}
 	default:
-		INTERNAL("Invalid ID %d in window_get_property().", JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case
+		 * and leave *@vp unchanged.  Do the same here.
+		 * (Actually not quite the same, as we already used
+		 * @undef_to_jsval.)  */
 		break;
 	}
 
@@ -278,7 +283,10 @@ window_set_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 		print_screen_status(vs->doc_view->session);
 		return JS_TRUE;
 	default:
-		INTERNAL("Invalid ID %d in window_set_property().", JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case.
+		 * Do the same here.  */
 		return JS_TRUE;
 	}
 
diff --git a/src/scripting/smjs/bookmarks.c b/src/scripting/smjs/bookmarks.c
index fb69930a6..3f0ca06fe 100644
--- a/src/scripting/smjs/bookmarks.c
+++ b/src/scripting/smjs/bookmarks.c
@@ -111,11 +111,14 @@ bookmark_get_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 
 		return JS_TRUE;
 	default:
-		INTERNAL("Invalid ID %d in bookmark_get_property().",
-		         JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case
+		 * and leave *@vp unchanged.  Do the same here.
+		 * (Actually not quite the same, as we already used
+		 * @undef_to_jsval.)  */
+		return JS_TRUE;
 	}
-
-	return JS_FALSE;
 }
 
 /* @bookmark_class.setProperty */
@@ -155,11 +158,12 @@ bookmark_set_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 		return JS_TRUE;
 	}
 	default:
-		INTERNAL("Invalid ID %d in bookmark_set_property().",
-		         JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case.
+		 * Do the same here.  */
+		return JS_TRUE;
 	}
-
-	return JS_FALSE;
 }
 
 static const JSClass bookmark_class = {
diff --git a/src/scripting/smjs/cache_object.c b/src/scripting/smjs/cache_object.c
index f621bdda4..fc3a21eb3 100644
--- a/src/scripting/smjs/cache_object.c
+++ b/src/scripting/smjs/cache_object.c
@@ -86,11 +86,14 @@ cache_entry_get_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 
 		return JS_TRUE;
 	default:
-		INTERNAL("Invalid ID %d in cache_entry_get_property().",
-		         JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case
+		 * and leave *@vp unchanged.  Do the same here.
+		 * (Actually not quite the same, as we already used
+		 * @undef_to_jsval.)  */
+		return JS_TRUE;
 	}
-
-	return JS_FALSE;
 }
 
 /* @cache_entry_class.setProperty */
@@ -140,13 +143,12 @@ cache_entry_set_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 		return JS_TRUE;
 	}
 	default:
-		INTERNAL("Invalid ID %d in cache_entry_set_property().",
-		         JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case.
+		 * Do the same here.  */
+		return JS_TRUE;
 	}
-
-
-
-	return JS_FALSE;
 }
 
 /* @cache_entry_class.finalize */
diff --git a/src/scripting/smjs/globhist.c b/src/scripting/smjs/globhist.c
index 21fa1f8bc..e46c643c0 100644
--- a/src/scripting/smjs/globhist.c
+++ b/src/scripting/smjs/globhist.c
@@ -97,11 +97,14 @@ smjs_globhist_item_get_property(JSContext *ctx, JSObject *obj, jsval id,
 
 		return JS_TRUE;
 	default:
-		INTERNAL("Invalid ID %d in globhist_get_property().",
-		         JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case
+		 * and leave *@vp unchanged.  Do the same here.
+		 * (Actually not quite the same, as we already used
+		 * @undef_to_jsval.)  */
+		return JS_TRUE;
 	}
-
-	return JS_FALSE;
 }
 
 /* @smjs_globhist_item_class.setProperty */
@@ -149,11 +152,12 @@ smjs_globhist_item_set_property(JSContext *ctx, JSObject *obj, jsval id, jsval *
 		return JS_TRUE;
 	}
 	default:
-		INTERNAL("Invalid ID %d in bookmark_set_property().",
-		         JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case.
+		 * Do the same here.  */
+		return JS_TRUE;
 	}
-
-	return JS_FALSE;
 }
 
 static const JSClass smjs_globhist_item_class = {
diff --git a/src/scripting/smjs/view_state_object.c b/src/scripting/smjs/view_state_object.c
index 3637f6c15..bc3926bb3 100644
--- a/src/scripting/smjs/view_state_object.c
+++ b/src/scripting/smjs/view_state_object.c
@@ -63,11 +63,14 @@ view_state_get_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 
 		return JS_TRUE;
 	default:
-		INTERNAL("Invalid ID %d in view_state_get_property().",
-		         JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case
+		 * and leave *@vp unchanged.  Do the same here.
+		 * (Actually not quite the same, as we already used
+		 * @undef_to_jsval.)  */
+		return JS_TRUE;
 	}
-
-	return JS_FALSE;
 }
 
 /* @view_state_class.setProperty */
@@ -94,11 +97,12 @@ view_state_set_property(JSContext *ctx, JSObject *obj, jsval id, jsval *vp)
 		return JS_TRUE;
 	}
 	default:
-		INTERNAL("Invalid ID %d in view_state_set_property().",
-		         JSVAL_TO_INT(id));
+		/* Unrecognized property ID; someone is using the
+		 * object as an array.  SMJS builtin classes (e.g.
+		 * js_RegExpClass) just return JS_TRUE in this case.
+		 * Do the same here.  */
+		return JS_TRUE;
 	}
-
-	return JS_FALSE;
 }
 
 static const JSClass view_state_class = {