diff --git a/src/Length.cpp b/src/Length.cpp
index f8eb920c0f..bb9ec6a3b9 100644
--- a/src/Length.cpp
+++ b/src/Length.cpp
@@ -19,6 +19,7 @@
 #include "LyXRC.h"
 
 #include "support/docstream.h"
+#include "support/lassert.h"
 
 #include <sstream>
 #include <iomanip>
@@ -67,7 +68,8 @@ void Length::swap(Length & rhs)
 string const Length::asString() const
 {
 	ostringstream os;
-	os << val_ << unit_name[unit_]; // setw?
+	if (unit_ != UNIT_NONE)
+		os << val_ << unit_name[unit_]; // setw?
 	return os.str();
 }
 
@@ -75,7 +77,8 @@ string const Length::asString() const
 docstring const Length::asDocstring() const
 {
 	odocstringstream os;
-	os << val_ << unit_name[unit_]; // setw?
+	if (unit_ != UNIT_NONE)
+		os << val_ << unit_name[unit_]; // setw?
 	return os.str();
 }
 
@@ -102,6 +105,9 @@ string const Length::asLatexString() const
 	case PPH:
 		os << val_ / 100.0 << "\\paperheight";
 		break;
+	case UNIT_NONE:
+		// One should not try to ouput latex code for an empty length
+		LASSERT(false, break);
 	default:
 		os << val_ << unit_name[unit_];
 	  break;
@@ -363,6 +369,9 @@ GlueLength::GlueLength(string const & data)
 
 string const GlueLength::asString() const
 {
+	if (len_.empty())
+		return string();
+
 	ostringstream buffer;
 
 	buffer << len_.value();
diff --git a/src/lengthcommon.cpp b/src/lengthcommon.cpp
index 217c22b604..72f5bbf88f 100644
--- a/src/lengthcommon.cpp
+++ b/src/lengthcommon.cpp
@@ -236,8 +236,11 @@ bool isValidGlueLength(string const & data, GlueLength * result)
 	// forward approach leads to very long, tedious code that would be
 	// much harder to understand and maintain. (AS)
 
-	if (data.empty())
+	if (data.empty()) {
+		if (result)
+			*result = GlueLength();
 		return true;
+	}
 	string buffer = ltrim(data);
 
 	// To make isValidGlueLength recognize negative values as
@@ -306,8 +309,11 @@ bool isValidLength(string const & data, Length * result)
 	// The parser may seem overkill for lengths without
 	// glue, but since we already have it, using it is
 	// easier than writing something from scratch.
-	if (data.empty())
+	if (data.empty()) {
+		if (result)
+			*result = Length();
 		return true;
+	}
 
 	string   buffer = data;
 	int      pattern_index = 0;