diff --git a/src/modules/graphics/opengl/GLBuffer.cpp b/src/modules/graphics/opengl/GLBuffer.cpp
index 416edee1e..841eaa20a 100644
--- a/src/modules/graphics/opengl/GLBuffer.cpp
+++ b/src/modules/graphics/opengl/GLBuffer.cpp
@@ -163,7 +163,7 @@ void GLBuffer::setMappedRangeModified(size_t offset, size_t modifiedsize)
 	// from the start of section a to the end of section b as modified if both
 	// a and b are marked as modified.
 
-	size_t old_range_end = modified_offset + modified_offset;
+	size_t old_range_end = modified_offset + modified_size;
 
 	modified_offset = std::min(modified_offset, offset);