--- a/dom/media/webspeech/synth/windows/SapiService.cpp
+++ b/dom/media/webspeech/synth/windows/SapiService.cpp
@@ -364,17 +364,18 @@ SapiService::Speak(const nsAString& aTex
   long rate = aRate != 0 ? static_cast<long>(10 * log10(aRate) / log10(3)) : 0;
   if (FAILED(spVoice->SetRate(rate))) {
     return NS_ERROR_FAILURE;
   }
 
   // Set the pitch using xml
   nsAutoString xml;
   xml.AssignLiteral("<pitch absmiddle=\"");
-  xml.AppendFloat(aPitch * 10.0f - 10.0f);
+  // absmiddle doesn't allow float type
+  xml.AppendInt(static_cast<int32_t>(aPitch * 10.0f - 10.0f));
   xml.AppendLiteral("\">");
   uint32_t textOffset = xml.Length();
 
   const char16_t* escapedText =
     nsEscapeHTML2(aText.BeginReading(), aText.Length());
   if (!escapedText) {
     return NS_ERROR_OUT_OF_MEMORY;
   }