diff --git a/src/cmd/compile/internal/gc/closure.go b/src/cmd/compile/internal/gc/closure.go
index 41ee74b8e0..9f0c073454 100644
--- a/src/cmd/compile/internal/gc/closure.go
+++ b/src/cmd/compile/internal/gc/closure.go
@@ -362,10 +362,6 @@ func transformclosure(xfunc *Node) {
 			xfunc.Func.Dcl = append(decls, xfunc.Func.Dcl...)
 		}
 
-		// Recalculate param offsets.
-		if f.Type.Width > 0 {
-			Fatalf("transformclosure: width is already calculated")
-		}
 		dowidth(f.Type)
 		xfunc.Type = f.Type // update type of ODCLFUNC
 	} else {
diff --git a/src/cmd/compile/internal/gc/type.go b/src/cmd/compile/internal/gc/type.go
index 49d222507b..772894f076 100644
--- a/src/cmd/compile/internal/gc/type.go
+++ b/src/cmd/compile/internal/gc/type.go
@@ -864,6 +864,15 @@ func (t *Type) FieldSlice() []*Field {
 
 // SetFields sets struct/interface type t's fields/methods to fields.
 func (t *Type) SetFields(fields []*Field) {
+	// If we've calculated the width of t before,
+	// then some other type such as a function signature
+	// might now have the wrong type.
+	// Rather than try to track and invalidate those,
+	// enforce that SetFields cannot be called once
+	// t's width has been calculated.
+	if t.WidthCalculated() {
+		Fatalf("SetFields of %v: width previously calculated", t)
+	}
 	t.wantEtype(TSTRUCT)
 	for _, f := range fields {
 		// If type T contains a field F with a go:notinheap