diff --git a/grovel/common.h b/grovel/common.h
index 4ea2aac99e486e726af65edd7dd550de002c227d..b44b0c24a78e90254e57e91a8b545d4cce36d02e 100644
--- a/grovel/common.h
+++ b/grovel/common.h
@@ -7,7 +7,10 @@
 #ifndef offsetof
 #define offsetof(type, slot) ((long) ((char *) &(((type *) 0)->slot)))
 #endif
-#define sizeofslot(type, slot) (sizeof(((type *) 0)->slot))
+#define getslot(type, slot) (((type *) 0)->slot)
+#define sizeofslot(type, slot) (sizeof(getslot(type, slot)))
+#define countofslot(type, slot) \
+  (sizeof(getslot(type, slot)) / sizeof(getslot(type, slot)[0]))
 #define stringify(x) #x
 #define indirect_stringify(x) stringify(x)
 
diff --git a/grovel/grovel.lisp b/grovel/grovel.lisp
index 62cf04633ec2a2aed985e82e99b953885a0adcb7..b7d2579ba29cbddb90580af5f62e989c3364978c 100644
--- a/grovel/grovel.lisp
+++ b/grovel/grovel.lisp
@@ -501,8 +501,7 @@ int main(int argc, char**argv) {
            (c-format out " :count ~D" count))
           ((eql :auto)
            (c-printf out " :count %i"
-                     (format nil "sizeof(~A) - offsetof(~A, ~A)"
-                             struct-c-name
+                     (format nil "countofslot(~A, ~A)"
                              struct-c-name
                              slot-c-name)))
           ((or symbol string)