diff --git a/sci-mathematics/mathmod/files/mathmod-13.0-fix_comparison.patch b/sci-mathematics/mathmod/files/mathmod-13.0-fix_comparison.patch
new file mode 100644
index 0000000000000..53a74cef6f052
--- /dev/null
+++ b/sci-mathematics/mathmod/files/mathmod-13.0-fix_comparison.patch
@@ -0,0 +1,24 @@
+backport https://github.com/parisolab/mathmod/commit/ddf9239f10fb0e07603297c06a23b7adeae8e323.patch
+see also https://github.com/parisolab/mathmod/issues/303
+fix comparison for clang
+--- a/ui_modules/mathmod.cpp
++++ b/ui_modules/mathmod.cpp
+@@ -1665,7 +1665,7 @@ void MathMod::CopyData(ObjectProperties *scene)
+     {
+         glBindBuffer(GL_ARRAY_BUFFER, vbo[0]);
+         glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, vbo[1]);
+-        if(scene->VertxNumber>previousVertxNumber>0)
++        if((scene->VertxNumber>previousVertxNumber) && (scene->VertxNumber>0))
+         {
+             glBufferData(GL_ARRAY_BUFFER, sizeof(GL_FLOAT)*10*(scene->VertxNumber+(12+60+36)), scene->ArrayNorVer_localPt, GL_STATIC_DRAW);
+             previousVertxNumber = scene->VertxNumber;
+@@ -1676,7 +1676,8 @@ void MathMod::CopyData(ObjectProperties *scene)
+             previousVertxNumber = scene->VertxNumber;
+         }
+ 
+-        if((scene->PolyNumber + scene->NbPolygnNbVertexPtMinSize)>previousPolyNumberNbPolygnNbVertexPtMin)
++        if(((scene->PolyNumber + scene->NbPolygnNbVertexPtMinSize)>previousPolyNumberNbPolygnNbVertexPtMin) &&
++            ((scene->PolyNumber + scene->NbPolygnNbVertexPtMinSize)>0))
+         {
+             glBufferData(GL_ELEMENT_ARRAY_BUFFER,sizeof(GLuint)*(scene->PolyNumber + scene->NbPolygnNbVertexPtMinSize), scene->PolyIndices_localPt, GL_STATIC_DRAW);
+             previousPolyNumberNbPolygnNbVertexPtMin =  (scene->PolyNumber + scene->NbPolygnNbVertexPtMinSize);
diff --git a/sci-mathematics/mathmod/mathmod-13.0-r1.ebuild b/sci-mathematics/mathmod/mathmod-13.0-r1.ebuild
index b7ab90667be80..c09e64d863365 100644
--- a/sci-mathematics/mathmod/mathmod-13.0-r1.ebuild
+++ b/sci-mathematics/mathmod/mathmod-13.0-r1.ebuild
@@ -1,4 +1,4 @@
-# Copyright 1999-2025 Gentoo Authors
+# Copyright 1999-2026 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 EAPI=8
@@ -22,9 +22,10 @@ RDEPEND="
 DEPEND="${RDEPEND}"
 
 PATCHES=(
-	# both merged
+	# all in master
 	"${FILESDIR}"/${PN}-13.0-missing_include.patch
 	"${FILESDIR}"/${PN}-13.0-fix_cxx20.patch
+	"${FILESDIR}"/${PN}-13.0-fix_comparison.patch
 )
 
 src_configure() {