diff --git a/string/public/nsAReadableString.h b/string/public/nsAReadableString.h
index 6bd72e921df..c21b7f7ed56 100644
--- a/string/public/nsAReadableString.h
+++ b/string/public/nsAReadableString.h
@@ -1230,8 +1230,8 @@ typedef basic_nsAReadableString<char>       nsAReadableCString;
 typedef basic_nsLiteralString<PRUnichar>    nsLiteralString;
 typedef basic_nsLiteralString<char>         nsLiteralCString;
 
-#define NS_LITERAL_STRING(s)  nsLiteralString(s, sizeof(s)/sizeof(wchar_t))
-#define NS_LITERAL_CSTRING(s) nsLiteralCString(s, sizeof(s))
+#define NS_LITERAL_STRING(s)  nsLiteralString(s, (sizeof(s)/sizeof(wchar_t))-1)
+#define NS_LITERAL_CSTRING(s) nsLiteralCString(s, sizeof(s)-1)
 
 typedef basic_nsLiteralChar<char>       nsLiteralChar;
 typedef basic_nsLiteralChar<PRUnichar>  nsLiteralPRUnichar;
diff --git a/xpcom/ds/nsAReadableString.h b/xpcom/ds/nsAReadableString.h
index 6bd72e921df..c21b7f7ed56 100644
--- a/xpcom/ds/nsAReadableString.h
+++ b/xpcom/ds/nsAReadableString.h
@@ -1230,8 +1230,8 @@ typedef basic_nsAReadableString<char>       nsAReadableCString;
 typedef basic_nsLiteralString<PRUnichar>    nsLiteralString;
 typedef basic_nsLiteralString<char>         nsLiteralCString;
 
-#define NS_LITERAL_STRING(s)  nsLiteralString(s, sizeof(s)/sizeof(wchar_t))
-#define NS_LITERAL_CSTRING(s) nsLiteralCString(s, sizeof(s))
+#define NS_LITERAL_STRING(s)  nsLiteralString(s, (sizeof(s)/sizeof(wchar_t))-1)
+#define NS_LITERAL_CSTRING(s) nsLiteralCString(s, sizeof(s)-1)
 
 typedef basic_nsLiteralChar<char>       nsLiteralChar;
 typedef basic_nsLiteralChar<PRUnichar>  nsLiteralPRUnichar;
diff --git a/xpcom/string/public/nsAReadableString.h b/xpcom/string/public/nsAReadableString.h
index 6bd72e921df..c21b7f7ed56 100644
--- a/xpcom/string/public/nsAReadableString.h
+++ b/xpcom/string/public/nsAReadableString.h
@@ -1230,8 +1230,8 @@ typedef basic_nsAReadableString<char>       nsAReadableCString;
 typedef basic_nsLiteralString<PRUnichar>    nsLiteralString;
 typedef basic_nsLiteralString<char>         nsLiteralCString;
 
-#define NS_LITERAL_STRING(s)  nsLiteralString(s, sizeof(s)/sizeof(wchar_t))
-#define NS_LITERAL_CSTRING(s) nsLiteralCString(s, sizeof(s))
+#define NS_LITERAL_STRING(s)  nsLiteralString(s, (sizeof(s)/sizeof(wchar_t))-1)
+#define NS_LITERAL_CSTRING(s) nsLiteralCString(s, sizeof(s)-1)
 
 typedef basic_nsLiteralChar<char>       nsLiteralChar;
 typedef basic_nsLiteralChar<PRUnichar>  nsLiteralPRUnichar;