diff --git a/filter/decl.m4 b/filter/decl.m4
index 71ac8803..a5cead75 100644
--- a/filter/decl.m4
+++ b/filter/decl.m4
@@ -143,7 +143,7 @@ FID_IFCONST([[
 }
 FID_IFCONST([[
   const struct f_inst **items = NULL;
-  if (constargs) {
+  if (constargs && whati->varcount) {
     items = alloca(whati->varcount * sizeof(struct f_inst *));
     const struct f_inst *child = fvar;
     for (uint i=0; child; i++)