diff --git a/layout/style/ServoBindings.cpp b/layout/style/ServoBindings.cpp
index 5d0e472fad40..149cbeeae445 100644
--- a/layout/style/ServoBindings.cpp
+++ b/layout/style/ServoBindings.cpp
@@ -1091,6 +1091,10 @@ Gecko_NewCSSValueSharedList(uint32_t aLen)
 void
 Gecko_CSSValue_SetAbsoluteLength(nsCSSValueBorrowedMut aCSSValue, nscoord aLen)
 {
+  MOZ_ASSERT(aCSSValue->GetUnit() == eCSSUnit_Null || aCSSValue->IsLengthUnit());
+  // The call below could trigger refcounting if aCSSValue were a
+  // FontFamilyList, but we just asserted that it's not. So we can
+  // whitelist this for static analysis.
   aCSSValue->SetIntegerCoordValue(aLen);
 }