diff --git a/src/util/StringBuffer.hxx b/src/util/StringBuffer.hxx
index 817e31063..49593d6c1 100644
--- a/src/util/StringBuffer.hxx
+++ b/src/util/StringBuffer.hxx
@@ -53,7 +53,7 @@ protected:
 public:
 	using const_iterator = typename Array::const_iterator;
 
-	constexpr size_type capacity() const noexcept {
+	static constexpr size_type capacity() noexcept {
 		return CAPACITY;
 	}