diff --git a/mailnews/extensions/bayesian-spam-filter/src/nsBayesianFilter.cpp b/mailnews/extensions/bayesian-spam-filter/src/nsBayesianFilter.cpp
index 6fdb3054b0ac..376534ad8aa9 100644
--- a/mailnews/extensions/bayesian-spam-filter/src/nsBayesianFilter.cpp
+++ b/mailnews/extensions/bayesian-spam-filter/src/nsBayesianFilter.cpp
@@ -193,7 +193,7 @@ static bool isDecimalNumber(const char* word)
     return true;
 }
 
-inline isUpperCase(char c) { return ('A' <= c) && (c <= 'Z'); }
+inline bool isUpperCase(char c) { return ('A' <= c) && (c <= 'Z'); }
 
 static char* toLowerCase(char* str)
 {