diff --git a/src/regexp.c b/src/regexp.c
index c95795d..dd99607 100644
--- a/src/regexp.c
+++ b/src/regexp.c
@@ -1319,7 +1319,7 @@
 	return NULL;
 
     /* Allocate space. */
-    r = alloc(sizeof(bt_regprog_T) + regsize);
+    r = alloc(offsetof(bt_regprog_T, program) + regsize);
     if (r == NULL)
 	return NULL;
     r->re_in_use = FALSE;