diff --git a/src/racket/src/mzclpf_post.inc b/src/racket/src/mzclpf_post.inc
index 89dd81740a..55838e3a3c 100644
--- a/src/racket/src/mzclpf_post.inc
+++ b/src/racket/src/mzclpf_post.inc
@@ -10,55 +10,58 @@
   if (data) {
     /* GLOBAL ASSUMPTION: prefix is at the end of a closure */
     Scheme_Prefix *pf = (Scheme_Prefix *)c->vals[closure_size - 1];
-    /* Since pf hasn't been marked, we don't need a GC_resolve(): */
-    int *use_bits = PREFIX_TO_USE_BITS(pf);
-    uintptr_t map;
+    
+    if (pf) {
+      /* Since pf hasn't been marked, we don't need a GC_resolve(): */
+      int *use_bits = PREFIX_TO_USE_BITS(pf);
+      uintptr_t map;
 
-    if (!pf->next_final) {
-      /* We're the first to look at this prefix... */
-      if (pf->num_stxes) {
-        /* Mark all syntax-object references */
-        for (i = pf->num_stxes+1; i--;) {
-          gcMARK2(pf->a[i+pf->num_toplevels], gc);
-        }
-      }
-      /* Add it to the chain of prefixes to finish after
-         all other marking: */
-      pf->next_final = scheme_prefix_finalize;
-      scheme_prefix_finalize = pf;
-    }
-
-    /* Mark just the elements of the prefix that are (newly) used: */
-    if ((uintptr_t)data->tl_map & 0x1) {
-      map = ((uintptr_t)data->tl_map) >> 1;
-      for (i = 0; i < 31; i++) {
-        if (map & (1 << i)) {
-          if (!(use_bits[0] & (1 << i))) {
-            if ((i < pf->num_toplevels) || !pf->num_stxes)
-              gcMARK2(pf->a[i], gc);
-            else
-              gcMARK2(pf->a[i + pf->num_stxes + 1], gc);
+      if (!pf->next_final) {
+        /* We're the first to look at this prefix... */
+        if (pf->num_stxes) {
+          /* Mark all syntax-object references */
+          for (i = pf->num_stxes+1; i--;) {
+            gcMARK2(pf->a[i+pf->num_toplevels], gc);
           }
         }
+        /* Add it to the chain of prefixes to finish after
+           all other marking: */
+        pf->next_final = scheme_prefix_finalize;
+        scheme_prefix_finalize = pf;
       }
-      use_bits[0] |= (map & 0x7FFFFFFF);
-    } else {
-      int *u = (int *)GC_resolve2(data->tl_map, gc), j, pos;
-      
-      for (i = u[0]; i--; ) {
-        map = u[i+1];
-        for (j = 0; j < 32; j++) {
-          if (map & (1 << j)) {
-            if (!(use_bits[i] & (1 << j))) {
-              pos = (i * 32) + j;
-              if ((pos < pf->num_toplevels) || !pf->num_stxes)
-                gcMARK2(pf->a[pos], gc);
+
+      /* Mark just the elements of the prefix that are (newly) used: */
+      if ((uintptr_t)data->tl_map & 0x1) {
+        map = ((uintptr_t)data->tl_map) >> 1;
+        for (i = 0; i < 31; i++) {
+          if (map & (1 << i)) {
+            if (!(use_bits[0] & (1 << i))) {
+              if ((i < pf->num_toplevels) || !pf->num_stxes)
+                gcMARK2(pf->a[i], gc);
               else
-                gcMARK2(pf->a[pos + pf->num_stxes + 1], gc);
+                gcMARK2(pf->a[i + pf->num_stxes + 1], gc);
             }
           }
         }
-        use_bits[i] |= map;
+        use_bits[0] |= (map & 0x7FFFFFFF);
+      } else {
+        int *u = (int *)GC_resolve2(data->tl_map, gc), j, pos;
+      
+        for (i = u[0]; i--; ) {
+          map = u[i+1];
+          for (j = 0; j < 32; j++) {
+            if (map & (1 << j)) {
+              if (!(use_bits[i] & (1 << j))) {
+                pos = (i * 32) + j;
+                if ((pos < pf->num_toplevels) || !pf->num_stxes)
+                  gcMARK2(pf->a[pos], gc);
+                else
+                  gcMARK2(pf->a[pos + pf->num_stxes + 1], gc);
+              }
+            }
+          }
+          use_bits[i] |= map;
+        }
       }
     }
   }