diff --git a/src/regexp_nfa.c b/src/regexp_nfa.c
index 92475ad..cc29a4c 100644
--- a/src/regexp_nfa.c
+++ b/src/regexp_nfa.c
@@ -7505,7 +7505,7 @@
     post2nfa(postfix, post_ptr, TRUE);
 
     // allocate the regprog with space for the compiled regexp
-    prog_size = sizeof(nfa_regprog_T) + sizeof(nfa_state_T) * (nstate - 1);
+    prog_size = offsetof(nfa_regprog_T, state) + sizeof(nfa_state_T) * nstate;
     prog = alloc(prog_size);
     if (prog == NULL)
 	goto fail;