diff --git a/misc/cgo/test/callback_c_gc.c b/misc/cgo/test/callback_c_gc.c
index 32bfed0c02..28a62c6dbc 100644
--- a/misc/cgo/test/callback_c_gc.c
+++ b/misc/cgo/test/callback_c_gc.c
@@ -39,6 +39,16 @@ callCgoAllocate(void)
 	int i;
 	struct { size_t n; void *ret; } a;
 	List *l, *head, **tail;
+
+	// Make sure this doesn't crash.
+	// And make sure it returns non-nil.
+	a.n = 0;
+	a.ret = 0;
+	crosscall2(_cgo_allocate, &a, sizeof a);
+	if(a.ret == 0) {
+		fprintf(stderr, "callCgoAllocate: alloc 0 returned nil\n");
+		exit(2);
+	}
 	
 	head = 0;
 	tail = &head;
diff --git a/misc/cgo/test/callback_c_gccgo.c b/misc/cgo/test/callback_c_gccgo.c
index d92dca0093..d367b7b68b 100644
--- a/misc/cgo/test/callback_c_gccgo.c
+++ b/misc/cgo/test/callback_c_gccgo.c
@@ -35,6 +35,13 @@ callCgoAllocate(void)
 	int i;
 	List *l, *head, **tail;
 	
+	// Make sure this doesn't crash.
+	// And make sure it returns non-nil.
+	if(_cgo_allocate(0) == 0) {
+		fprintf(stderr, "callCgoAllocate: alloc 0 returned nil\n");
+		exit(2);
+	}
+
 	head = 0;
 	tail = &head;
 	for(i=0; i<100; i++) {
diff --git a/src/runtime/cgocallback.go b/src/runtime/cgocallback.go
index 1e1b576072..2c89143208 100644
--- a/src/runtime/cgocallback.go
+++ b/src/runtime/cgocallback.go
@@ -21,6 +21,9 @@ import "unsafe"
 // Either we need to add types or we need to stop using it.
 
 func _cgo_allocate_internal(len uintptr) unsafe.Pointer {
+	if len == 0 {
+		len = 1
+	}
 	ret := unsafe.Pointer(&make([]unsafe.Pointer, (len+ptrSize-1)/ptrSize)[0])
 	c := new(cgomal)
 	c.alloc = ret