diff --git a/src/cmd/gc/go.h b/src/cmd/gc/go.h
index 73deb98263a..72c6b745d4c 100644
--- a/src/cmd/gc/go.h
+++ b/src/cmd/gc/go.h
@@ -470,6 +470,7 @@ enum
 	Etype = 1<<3,
 	Ecall = 1<<4,	// call-only expressions are ok
 	Efnstruct = 1<<5,	// multivalue function returns are ok
+	Eiota = 1<<6,		// iota is ok
 };
 
 #define	BITS	5
diff --git a/src/cmd/gc/typecheck.c b/src/cmd/gc/typecheck.c
index 983ff78356a..a7d5b8d40e5 100644
--- a/src/cmd/gc/typecheck.c
+++ b/src/cmd/gc/typecheck.c
@@ -85,6 +85,8 @@ reswitch:
 	 */
 	case OLITERAL:
 		ok |= Erv;
+		if(n->iota && !(top & Eiota))
+			yyerror("use of iota outside of constant initializer");
 		goto ret;
 
 	case ONONAME:
@@ -261,8 +263,8 @@ reswitch:
 	case OSUB:
 	case OXOR:
 		ok |= Erv;
-		l = typecheck(&n->left, Erv);
-		r = typecheck(&n->right, Erv);
+		l = typecheck(&n->left, Erv | (top & Eiota));
+		r = typecheck(&n->right, Erv | (top & Eiota));
 		if(l->type == T || r->type == T)
 			goto error;
 		op = n->op;
@@ -339,7 +341,7 @@ reswitch:
 	case ONOT:
 	case OPLUS:
 		ok |= Erv;
-		l = typecheck(&n->left, Erv);
+		l = typecheck(&n->left, Erv | (top & Eiota));
 		if((t = l->type) == T)
 			goto error;
 		if(!okfor[n->op][t->etype]) {
@@ -995,6 +997,8 @@ error:
 out:
 	lineno = lno;
 	n->typecheck = 1;
+	if(n->iota)
+		n->typecheck = 0;
 	*np = n;
 	return n;
 }
diff --git a/src/cmd/gc/walk.c b/src/cmd/gc/walk.c
index d1e7f3a1bea..fd1220f6186 100644
--- a/src/cmd/gc/walk.c
+++ b/src/cmd/gc/walk.c
@@ -160,7 +160,7 @@ walkdef(Node *n)
 			dump("walkdef nil defn", n);
 			yyerror("xxx");
 		}
-		typecheck(&e, Erv);
+		typecheck(&e, Erv | Eiota);
 		if(e->op != OLITERAL) {
 			yyerror("const initializer must be constant");
 			goto ret;
diff --git a/test/fixedbugs/bug186.go b/test/fixedbugs/bug186.go
new file mode 100644
index 00000000000..97c094734d8
--- /dev/null
+++ b/test/fixedbugs/bug186.go
@@ -0,0 +1,18 @@
+// errchk $G $D/$F.go
+
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+const X = iota
+
+func f(x int) { }
+
+func main() {
+	f(X);
+	f(iota);	// ERROR "iota.*outside.*initializer"
+	f(X);
+	f(iota);	// ERROR "iota.*outside.*initializer"
+}