--- a/dom/media/PeerConnection.js
+++ b/dom/media/PeerConnection.js
@@ -838,42 +838,56 @@ class RTCPeerConnection {
           let request = this._win.CreateOfferRequest._create(this._win, chrome);
           Services.obs.notifyObservers(request, "PeerConnection:request");
         });
       }
     }
     return this._havePermission;
   }
 
-  setLocalDescription(desc, onSucc, onErr) {
-    return this._auto(onSucc, onErr, () => this._setLocalDescription(desc));
-  }
-
-  async _setLocalDescription({ type, sdp }) {
-    this._checkClosed();
-
-    this._localType = type;
-
-    let action = this._actions[type];
+  _sanityCheckSdp(action, type, sdp) {
     if (action === undefined) {
       throw new this._win.DOMException(
           "Invalid type " + type + " provided to setLocalDescription",
           "InvalidParameterError");
     }
     if (action == Ci.IPeerConnection.kActionPRAnswer) {
       throw new this._win.DOMException("pranswer not yet implemented",
                                        "NotSupportedError");
     }
 
     if (!sdp && action != Ci.IPeerConnection.kActionRollback) {
       throw new this._win.DOMException(
           "Empty or null SDP provided to setLocalDescription",
           "InvalidParameterError");
     }
 
+    // The fippo butter finger filter AKA non-ASCII chars
+    // Note: SDP allows non-ASCII character in the subject (who cares?)
+    let pos = sdp.search(/[^\u0000-\u007f]/);
+    if (pos != -1) {
+      throw new this._win.DOMException(
+          "SDP contains non ASCII characters at position " + pos,
+          "InvalidParameterError");
+    }
+  }
+
+  setLocalDescription(desc, onSucc, onErr) {
+    return this._auto(onSucc, onErr, () => this._setLocalDescription(desc));
+  }
+
+  async _setLocalDescription({ type, sdp }) {
+    this._checkClosed();
+
+    this._localType = type;
+
+    let action = this._actions[type];
+
+    this._sanityCheckSdp(action, type, sdp);
+
     return this._chain(async () => {
       await this._getPermission();
       await new Promise((resolve, reject) => {
         this._onSetLocalDescriptionSuccess = resolve;
         this._onSetLocalDescriptionFailure = reject;
         this._impl.setLocalDescription(action, sdp);
       });
     });
@@ -928,31 +942,18 @@ class RTCPeerConnection {
     return this._auto(onSucc, onErr, () => this._setRemoteDescription(desc));
   }
 
   async _setRemoteDescription({ type, sdp }) {
     this._checkClosed();
     this._remoteType = type;
 
     let action = this._actions[type];
-    if (action === undefined) {
-      throw new this._win.DOMException(
-          "Invalid type " + type + " provided to setRemoteDescription",
-          "InvalidParameterError");
-    }
-    if (action == Ci.IPeerConnection.kActionPRAnswer) {
-      throw new this._win.DOMException("pranswer not yet implemented",
-                                       "NotSupportedError");
-    }
 
-    if (!sdp && type != "rollback") {
-      throw new this._win.DOMException(
-          "Empty or null SDP provided to setRemoteDescription",
-          "InvalidParameterError");
-    }
+    this._sanityCheckSdp(action, type, sdp);
 
     // Get caller's origin before hitting the promise chain
     let origin = Cu.getWebIDLCallerPrincipal().origin;
 
     return this._chain(async () => {
       let haveSetRemote = (async () => {
         await this._getPermission();
         await new Promise((resolve, reject) => {