diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index 037cd96..bde01d3 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -674,9 +674,7 @@ zstream_expand_buffer(struct zstream *z)
 	        rb_obj_reveal(z->buf, rb_cString);
             }
 
-            rb_mutex_unlock(z->mutex);
-	    rb_protect(rb_yield, z->buf, &state);
-            rb_mutex_lock(z->mutex);
+            rb_protect(rb_yield, z->buf, &state);
 
             if (ZSTREAM_REUSE_BUFFER_P(z)) {
                 rb_str_modify(z->buf);
@@ -729,6 +727,7 @@ zstream_expand_buffer_protect(struct zstream *z)
 
     return state;
 }
+
 static int
 zstream_expand_buffer_non_stream(struct zstream *z)
 {
@@ -1207,7 +1206,11 @@ zstream_run(struct zstream *z, Bytef *src, long len, int flush)
         .jump_state = 0,
         .stream_output = !ZSTREAM_IS_GZFILE(z) && rb_block_given_p(),
     };
-    rb_mutex_lock(z->mutex);
+
+    if (rb_mutex_trylock(z->mutex) == Qfalse) {
+        rb_raise(cInProgressError, "zlib stream is in progress");
+    }
+
     rb_ensure(zstream_run_try, (VALUE)&args, zstream_run_ensure, (VALUE)&args);
     if (args.jump_state)
 	rb_jump_tag(args.jump_state);