diff --git a/src/sirit.cpp b/src/sirit.cpp
index a2043ae..3c9d97d 100644
--- a/src/sirit.cpp
+++ b/src/sirit.cpp
@@ -42,8 +42,10 @@ std::vector<u32> Module::Assemble() const {
     }
 
     for (const std::string_view extension_name : extensions) {
+        size_t count = WordsInString(extension_name);
+        words.push_back(MakeWord0(spv::Op::OpExtension, count + 1));
         size_t insert_index = words.size();
-        words.resize(words.size() + WordsInString(extension_name));
+        words.resize(words.size() + count);
         InsertStringView(words, insert_index, extension_name);
     }
 
diff --git a/src/stream.h b/src/stream.h
index cbea1d4..fecc6d0 100644
--- a/src/stream.h
+++ b/src/stream.h
@@ -35,7 +35,7 @@ struct OpId {
 struct EndOp {};
 
 constexpr size_t WordsInString(std::string_view string) {
-    return string.size() / sizeof(u32);
+    return string.size() / sizeof(u32) + 1;
 }
 
 inline void InsertStringView(std::vector<u32>& words, size_t& insert_index,