diff --git a/bugs-fixed/README b/bugs-fixed/README
index e834a29..9c644f9 100644
--- a/bugs-fixed/README
+++ b/bugs-fixed/README
@@ -51,3 +51,4 @@ array passed as the second argument, then split() would previously read
 from the freed memory and possibly produce incorrect results (depending
 on the system's malloc()/free() behaviour.)
 
+
diff --git a/bugs-fixed/negative-nf.awk b/bugs-fixed/negative-nf.awk
new file mode 100644
index 0000000..6caeee4
--- /dev/null
+++ b/bugs-fixed/negative-nf.awk
@@ -0,0 +1 @@
+BEGIN { NF = -5; }
diff --git a/bugs-fixed/negative-nf.ok b/bugs-fixed/negative-nf.ok
new file mode 100644
index 0000000..71c8604
--- /dev/null
+++ b/bugs-fixed/negative-nf.ok
@@ -0,0 +1,2 @@
+./a.out: cannot set NF to a negative value
+ source line number 1
diff --git a/bugs-fixed/nf-self-assign.awk b/bugs-fixed/nf-self-assign.awk
new file mode 100644
index 0000000..6ae29ee
--- /dev/null
+++ b/bugs-fixed/nf-self-assign.awk
@@ -0,0 +1,6 @@
+BEGIN {
+	$0="a b c";
+	OFS=",";
+	NF = NF;
+	print;
+}
diff --git a/bugs-fixed/nf-self-assign.bad b/bugs-fixed/nf-self-assign.bad
new file mode 100644
index 0000000..3774da6
--- /dev/null
+++ b/bugs-fixed/nf-self-assign.bad
@@ -0,0 +1 @@
+a b c
diff --git a/bugs-fixed/nf-self-assign.ok b/bugs-fixed/nf-self-assign.ok
new file mode 100644
index 0000000..b2ffb02
--- /dev/null
+++ b/bugs-fixed/nf-self-assign.ok
@@ -0,0 +1 @@
+a,b,c
diff --git a/lib.c b/lib.c
index f09aeb2..f9cb6ca 100644
--- a/lib.c
+++ b/lib.c
@@ -393,6 +393,8 @@ void newfld(int n)	/* add field n after end of existing lastfld */
 
 void setlastfld(int n)	/* set lastfld cleaning fldtab cells if necessary */
 {
+	if (n < 0)
+		FATAL("cannot set NF to a negative value");
 	if (n > nfields)
 		growfldtab(n);
 
diff --git a/run.c b/run.c
index bf84b76..ce30e93 100644
--- a/run.c
+++ b/run.c
@@ -1123,8 +1123,8 @@ Cell *assign(Node **a, int n)	/* a[0] = a[1], a[0] += a[1], etc. */
 	y = execute(a[1]);
 	x = execute(a[0]);
 	if (n == ASSIGN) {	/* ordinary assignment */
-		if (x == y && !(x->tval & (FLD|REC)))	/* self-assignment: */
-			;		/* leave alone unless it's a field */
+		if (x == y && !(x->tval & (FLD|REC)) && x != nfloc)
+			;	/* self-assignment: leave alone unless it's a field or NF */
 		else if ((y->tval & (STR|NUM)) == (STR|NUM)) {
 			setsval(x, getsval(y));
 			x->fval = getfval(y);