diff --git a/libc/stdio/fmemopen.cpp b/libc/stdio/fmemopen.cpp
index 9d8c41f..6e333ba 100644
--- a/libc/stdio/fmemopen.cpp
+++ b/libc/stdio/fmemopen.cpp
@@ -149,7 +149,9 @@
   } else if (mode[0] == 'w') {
     ck->size = 0;
     ck->offset = 0;
-    ck->buf[0] = '\0';
+    if (capacity > 0) {
+      ck->buf[0] = '\0';
+    }
   }
 
   return fp;