diff --git a/src/writer/spirv/builder.cc b/src/writer/spirv/builder.cc
index 193584c..fc01258 100644
--- a/src/writer/spirv/builder.cc
+++ b/src/writer/spirv/builder.cc
@@ -2316,6 +2316,7 @@
     case ast::type::ImageFormat::kRgba32Float:
       return SpvImageFormatRgba32f;
   }
+  return SpvImageFormatUnknown;
 }
 
 }  // namespace spirv