diff --git a/src/pgfimage.cpp b/src/pgfimage.cpp
index 01eaab5c..8c62d3b6 100644
--- a/src/pgfimage.cpp
+++ b/src/pgfimage.cpp
@@ -215,7 +215,7 @@ void PgfImage::doWriteMetadata(BasicIo& outIo) {
 }  // PgfImage::doWriteMetadata
 
 byte PgfImage::readPgfMagicNumber(BasicIo& iIo) {
-  byte b = iIo.getb();
+  auto b = static_cast<byte>(iIo.getb());
   if (iIo.error())
     throw Error(ErrorCode::kerFailedToReadImageData);