diff --git a/base/src/nsString.cpp b/base/src/nsString.cpp
index 772eba83bca..0a3f4c9d43b 100644
--- a/base/src/nsString.cpp
+++ b/base/src/nsString.cpp
@@ -1910,6 +1910,7 @@ nsAutoString::nsAutoString()
   mStr = mBuf;
   mLength=0;
   mCapacity = (sizeof(mBuf) / sizeof(chartype))-sizeof(chartype);
+  mStr[0] = 0;
 }
 
 /*-------------------------------------------------------
diff --git a/string/obsolete/nsString.cpp b/string/obsolete/nsString.cpp
index 772eba83bca..0a3f4c9d43b 100644
--- a/string/obsolete/nsString.cpp
+++ b/string/obsolete/nsString.cpp
@@ -1910,6 +1910,7 @@ nsAutoString::nsAutoString()
   mStr = mBuf;
   mLength=0;
   mCapacity = (sizeof(mBuf) / sizeof(chartype))-sizeof(chartype);
+  mStr[0] = 0;
 }
 
 /*-------------------------------------------------------
diff --git a/xpcom/ds/nsString.cpp b/xpcom/ds/nsString.cpp
index 772eba83bca..0a3f4c9d43b 100644
--- a/xpcom/ds/nsString.cpp
+++ b/xpcom/ds/nsString.cpp
@@ -1910,6 +1910,7 @@ nsAutoString::nsAutoString()
   mStr = mBuf;
   mLength=0;
   mCapacity = (sizeof(mBuf) / sizeof(chartype))-sizeof(chartype);
+  mStr[0] = 0;
 }
 
 /*-------------------------------------------------------
diff --git a/xpcom/string/obsolete/nsString.cpp b/xpcom/string/obsolete/nsString.cpp
index 772eba83bca..0a3f4c9d43b 100644
--- a/xpcom/string/obsolete/nsString.cpp
+++ b/xpcom/string/obsolete/nsString.cpp
@@ -1910,6 +1910,7 @@ nsAutoString::nsAutoString()
   mStr = mBuf;
   mLength=0;
   mCapacity = (sizeof(mBuf) / sizeof(chartype))-sizeof(chartype);
+  mStr[0] = 0;
 }
 
 /*-------------------------------------------------------