diff --git a/src/frontends/qt/GuiFontLoader.cpp b/src/frontends/qt/GuiFontLoader.cpp
index cf6c205d2d..a808431d10 100644
--- a/src/frontends/qt/GuiFontLoader.cpp
+++ b/src/frontends/qt/GuiFontLoader.cpp
@@ -44,6 +44,15 @@ int const num_math_fonts = sizeof(math_fonts) / sizeof(*math_fonts);
 
 namespace lyx {
 
+bool isMathFamily(QString const & name)
+{
+	for (int i = 0; i < num_math_fonts; ++i) {
+		if (math_fonts[i] == name)
+			return true;
+	}
+	return false;
+}
+
 namespace frontend {
 
 /**
diff --git a/src/frontends/qt/GuiFontLoader.h b/src/frontends/qt/GuiFontLoader.h
index 40c15f5ea0..21e24a7ef4 100644
--- a/src/frontends/qt/GuiFontLoader.h
+++ b/src/frontends/qt/GuiFontLoader.h
@@ -13,11 +13,14 @@
 #define GUI_FONTLOADER_H
 
 class QFont;
+class QString;
 
 namespace lyx {
 
 class FontInfo;
 
+bool isMathFamily(QString const & name);
+
 namespace frontend {
 
 class GuiFontMetrics;
diff --git a/src/frontends/qt/GuiFontMetrics.cpp b/src/frontends/qt/GuiFontMetrics.cpp
index 631aecc062..f0300f16ea 100644
--- a/src/frontends/qt/GuiFontMetrics.cpp
+++ b/src/frontends/qt/GuiFontMetrics.cpp
@@ -11,6 +11,7 @@
 
 #include <config.h>
 
+#include "GuiFontLoader.h"
 #include "GuiFontMetrics.h"
 
 #include "qt_helpers.h"
@@ -276,10 +277,12 @@ int GuiFontMetrics::width(docstring const & s) const
 	*/
 	int w = 0;
 	// is the string a single character from a math font ?
-	bool const math_char = s.length() == 1 && font_.styleName() == "LyX";
+	// we have to also explicitly check for the family, see bug 13087
+	bool const math_char = s.length() == 1
+		&& (font_.styleName() == "LyX" || isMathFamily(font_.family()));
 	if (math_char) {
 		QString const qs = toqstr(s);
-		int br_width = metrics_.boundingRect(qs).width();
+		int br_width = rbearing(s[0]);
 #if QT_VERSION >= 0x050b00
 		int s_width = metrics_.horizontalAdvance(qs);
 #else