diff --git a/m4/mmap_write.m4 b/m4/mmap_write.m4
index 0aa0b61799..f1241a19d5 100644
--- a/m4/mmap_write.m4
+++ b/m4/mmap_write.m4
@@ -25,7 +25,7 @@ AC_DEFUN([DOVECOT_MMAP_WRITE], [
           perror("mmap()");
           return 1;
         }
-        strcpy(mem, "2");
+        strncpy(mem, "2");
         msync(mem, 2, MS_SYNC);
         lseek(f, 0, SEEK_SET);
         write(f, "3", 2);