diff --git a/src/bfu/inpfield.c b/src/bfu/inpfield.c
index 7857cd23..2d6ba4ca 100644
--- a/src/bfu/inpfield.c
+++ b/src/bfu/inpfield.c
@@ -595,9 +595,9 @@ kbd_field(struct dialog_data *dlg_data, struct widget_data *widget_data)
 				int cdata_len = strlen(widget_data->cdata);
 				int start = widget_data->info.field.cpos;
 
-				while (start > 0 && isspace(widget_data->cdata[start - 1]))
+				while (start > 0 && isspace((unsigned char)widget_data->cdata[start - 1]))
 					--start;
-				while (start > 0 && !isspace(widget_data->cdata[start - 1]))
+				while (start > 0 && !isspace((unsigned char)widget_data->cdata[start - 1]))
 					--start;
 
 				memmove(widget_data->cdata + start,
@@ -610,19 +610,19 @@ kbd_field(struct dialog_data *dlg_data, struct widget_data *widget_data)
 			}
 
 		case ACT_EDIT_MOVE_BACKWARD_WORD:
-			while (widget_data->info.field.cpos > 0 && isspace(widget_data->cdata[widget_data->info.field.cpos - 1]))
+			while (widget_data->info.field.cpos > 0 && isspace((unsigned char)widget_data->cdata[widget_data->info.field.cpos - 1]))
 				--widget_data->info.field.cpos;
-			while (widget_data->info.field.cpos > 0 && !isspace(widget_data->cdata[widget_data->info.field.cpos - 1]))
+			while (widget_data->info.field.cpos > 0 && !isspace((unsigned char)widget_data->cdata[widget_data->info.field.cpos - 1]))
 				--widget_data->info.field.cpos;
 
 			goto display_field;
 
 		case ACT_EDIT_MOVE_FORWARD_WORD:
-			while (isspace(widget_data->cdata[widget_data->info.field.cpos]))
+			while (isspace((unsigned char)widget_data->cdata[widget_data->info.field.cpos]))
 				++widget_data->info.field.cpos;
-			while (widget_data->cdata[widget_data->info.field.cpos] && !isspace(widget_data->cdata[widget_data->info.field.cpos]))
+			while (widget_data->cdata[widget_data->info.field.cpos] && !isspace((unsigned char)widget_data->cdata[widget_data->info.field.cpos]))
 				++widget_data->info.field.cpos;
-			while (isspace(widget_data->cdata[widget_data->info.field.cpos]))
+			while (isspace((unsigned char)widget_data->cdata[widget_data->info.field.cpos]))
 				++widget_data->info.field.cpos;
 
 			goto display_field;
diff --git a/src/bfu/text.c b/src/bfu/text.c
index 6794919b..1ba0ef66 100644
--- a/src/bfu/text.c
+++ b/src/bfu/text.c
@@ -20,7 +20,7 @@
 #include "util/color.h"
 
 /* FIXME: For UTF-8 strings we need better function than isspace. */
-#define is_unsplitable(pos) (*(pos) && *(pos) != '\n' && !isspace(*(pos)))
+#define is_unsplitable(pos) (*(pos) && *(pos) != '\n' && !isspace((unsigned char)*(pos)))
 
 void
 add_dlg_text(struct dialog *dlg, char *text,
@@ -175,7 +175,7 @@ split_lines(struct widget_data *widget_data, int max_width)
 		int cells = 0;
 
 		/* Skip first leading \n or space. */
-		if (isspace(*text)) text++;
+		if (isspace((unsigned char)*text)) text++;
 		if (!*text) break;
 
 #ifdef CONFIG_UTF8
@@ -229,7 +229,7 @@ dlg_format_text_do(struct dialog_data *dlg_data,
 		int cells = 0;
 
 		/* Skip first leading \n or space. */
-		if (!firstline && isspace(*text))
+		if (!firstline && isspace((unsigned char)*text))
 			text++;
 		else
 			firstline = 0;