diff --git a/src/output_docbook.cpp b/src/output_docbook.cpp
index f35b82e8f2..71d3b83f29 100644
--- a/src/output_docbook.cpp
+++ b/src/output_docbook.cpp
@@ -754,7 +754,7 @@ DocBookDocumentSectioning hasDocumentSectioning(ParagraphList const &paragraphs,
 	bool documentHasSections = false;
 
 	while (bpit < epit) {
-		LASSERT(bpit < paragraphs.size(), return make_tuple(documentHasSections, bpit));
+		LASSERT(static_cast<size_t>(bpit) < paragraphs.size(), return make_tuple(documentHasSections, bpit));
 
 		Layout const &style = paragraphs[bpit].layout();
 		documentHasSections |= isLayoutSectioningOrSimilar(style);