diff --git a/src/mathed/ChangeLog b/src/mathed/ChangeLog
index 502a39dd0a..09d22478e3 100644
--- a/src/mathed/ChangeLog
+++ b/src/mathed/ChangeLog
@@ -1,6 +1,11 @@
+2004-10-15  Georg Baum  <Georg.Baum@post.rwth-aachen.de>
+
+	* math_hullinset.C (mutate): fix endless loop for unknown types
+
 2004-10-05  Andreas Vox  <vox@isp.uni-luebeck.de>
 
-	* math_hullinset.C (docbook): write additional <alt role="tex" > tag for	Docbook  XML
+	* math_hullinset.C (docbook): write additional <alt role="tex" >
+	tag for	Docbook XML
 
 2004-09-21  Jean-Marc Lasgouttes  <lasgouttes@lyx.org>
 
diff --git a/src/mathed/math_hullinset.C b/src/mathed/math_hullinset.C
index 941c438624..5d068841c4 100644
--- a/src/mathed/math_hullinset.C
+++ b/src/mathed/math_hullinset.C
@@ -110,7 +110,7 @@ namespace {
 		if (s == "gather")    return 9;
 		if (s == "flalign")   return 10;
 		lyxerr << "unknown hull type '" << s << "'" << endl;
-		return 0;
+		return -1;
 	}
 
 	bool smaller(string const & s, string const & t)
@@ -700,6 +700,10 @@ void MathHullInset::mutate(string const & newtype)
 		// done
 	}
 
+	else if (typecode(newtype) < 0) {
+		// unknown type
+	}
+
 	else if (type_ == "none") {
 		setType("simple");
 		numbered(0, false);