diff --git a/src/codec/SkStreamBuffer.cpp b/src/codec/SkStreamBuffer.cpp
index 00253fa..e8a6e90 100644
--- a/src/codec/SkStreamBuffer.cpp
+++ b/src/codec/SkStreamBuffer.cpp
@@ -72,7 +72,8 @@
         return sk_ref_sp<SkData>(*data);
     }
 
-    SkASSERT(position + length <= fStream->getLength());
+    SkASSERT(length <= fStream->getLength() &&
+             position <= fStream->getLength() - length);
 
     const size_t oldPosition = fStream->getPosition();
     if (!fStream->seek(position)) {