diff --git a/src/evalfunc.c b/src/evalfunc.c
index 287ca3c..fd385fe 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -6689,9 +6689,14 @@
 	if (argvars[2].v_type == VAR_STRING)
 	    string_in = argvars[2].vval.v_string;
 	if (type == VAR_NUMBER)
+	{
 	    string_result = NULL;
+	}
 	else
+	{
+	    rettv->vval.v_string = NULL;
 	    string_result = &rettv->vval.v_string;
+	}
 	if (mch_libcall(argvars[0].vval.v_string,
 			     argvars[1].vval.v_string,
 			     string_in,