diff --git a/src/csg/CsgEvaluator.cpp b/src/csg/CsgEvaluator.cpp
index 2fcc799..1860e6a 100644
--- a/src/csg/CsgEvaluator.cpp
+++ b/src/csg/CsgEvaluator.cpp
@@ -1209,16 +1209,34 @@ CsgNodePtr CsgEvaluator::evalExtrusion(const ExtrusionNode& e, const glm::mat4&
         if (name == "scale") {
             Value sv = m_interp->evaluate(*exprPtr);
             if (sv.isNumber()) {
-                ext.params["scale_x"] = sv.asNumber();
-                ext.params["scale_y"] = sv.asNumber();
+                // A non-finite scale (1/0, -1/0, 0/0) is a real IEEE-754
+                // Number per evaluate()'s arithmetic propagation, so it'd
+                // sail past this isNumber() check — but unlike the generic
+                // param path below (which routes through evalNumber() and
+                // folds non-finite to 0.0), this branch used to assign the
+                // raw value straight into scale_x/scale_y. Manifold::Extrude
+                // then lerps every extruded vertex toward that inf/nan
+                // scale, producing corrupt geometry that free()s garbage
+                // heap metadata downstream (issue #105). Leaving scale_x/
+                // scale_y unset here falls back to MeshEvaluator's default
+                // 1.0 (no scaling), same as any other malformed scale value.
+                double s = sv.asNumber();
+                if (std::isfinite(s)) {
+                    ext.params["scale_x"] = s;
+                    ext.params["scale_y"] = s;
+                }
             } else if (sv.isVector() && sv.asVec().size() == 2) {
                 // Exactly 2 elements — anything else (e.g. a stray 3-vector)
                 // isn't a valid linear_extrude() scale and OpenSCAD ignores
                 // it entirely (falls back to no scaling), rather than using
                 // just the first two components — confirmed against real
                 // OpenSCAD's STL output (docs/roadmap.md v3.9).
-                ext.params["scale_x"] = sv.asVec()[0].asNumber();
-                ext.params["scale_y"] = sv.asVec()[1].asNumber();
+                double sx = sv.asVec()[0].asNumber();
+                double sy = sv.asVec()[1].asNumber();
+                if (std::isfinite(sx) && std::isfinite(sy)) {
+                    ext.params["scale_x"] = sx;
+                    ext.params["scale_y"] = sy;
+                }
             }
         } else if (name == "center") {
             Value cv = m_interp->evaluate(*exprPtr);
diff --git a/tests/test_csg_evaluator.cpp b/tests/test_csg_evaluator.cpp
index e84f622..53a0554 100644
--- a/tests/test_csg_evaluator.cpp
+++ b/tests/test_csg_evaluator.cpp
@@ -1560,6 +1560,54 @@ TEST_CASE("CsgEval:linear_extrude scale must be exactly a 2-vector, else no scal
     REQUIRE(good.params.at("scale_y") == Approx(5.0));
 }
 
+TEST_CASE("CsgEval:linear_extrude non-finite scale is rejected, not passed through",
+          "[csg][v105][bugfix]") {
+    // Issue #105: linear_extrude-parameter-tests.scad crashed with heap
+    // corruption ("free(): invalid next size (fast)"). Root cause: scale=
+    // is a real IEEE-754 Number for 1/0, -1/0, and 0/0 (evaluate()'s
+    // arithmetic propagates non-finite doubles), so it passed the
+    // sv.isNumber() check here — but unlike the generic param path (which
+    // routes through evalNumber() and folds non-finite to 0.0) and unlike
+    // "angle" (which is explicitly resolved to a finite fallback), this
+    // branch used to assign the raw inf/nan straight into scale_x/scale_y.
+    // MeshEvaluator/Manifold::Extrude then lerped every extruded vertex
+    // toward that non-finite scale, corrupting the heap. Non-finite scale
+    // must leave scale_x/scale_y unset (MeshEvaluator's default: no
+    // scaling), same as any other malformed scale value.
+    auto posInf = asExtrusion(
+        evaluate("linear_extrude(height=20, scale=1/0) square(10);").roots[0]);
+    REQUIRE_FALSE(posInf.params.count("scale_x"));
+    REQUIRE_FALSE(posInf.params.count("scale_y"));
+
+    auto negInf = asExtrusion(
+        evaluate("linear_extrude(height=20, scale=-1/0) square(10);").roots[0]);
+    REQUIRE_FALSE(negInf.params.count("scale_x"));
+    REQUIRE_FALSE(negInf.params.count("scale_y"));
+
+    auto nan = asExtrusion(
+        evaluate("linear_extrude(height=20, scale=0/0) square(10);").roots[0]);
+    REQUIRE_FALSE(nan.params.count("scale_x"));
+    REQUIRE_FALSE(nan.params.count("scale_y"));
+
+    // Same for a 2-vector with a non-finite component.
+    auto vecInf = asExtrusion(
+        evaluate("linear_extrude(height=20, scale=[1/0, 2]) square(10);").roots[0]);
+    REQUIRE_FALSE(vecInf.params.count("scale_x"));
+    REQUIRE_FALSE(vecInf.params.count("scale_y"));
+
+    // Symmetric case: non-finite in the second component only.
+    auto vecInf2 = asExtrusion(
+        evaluate("linear_extrude(height=20, scale=[2, 1/0]) square(10);").roots[0]);
+    REQUIRE_FALSE(vecInf2.params.count("scale_x"));
+    REQUIRE_FALSE(vecInf2.params.count("scale_y"));
+
+    // A finite scale is unaffected.
+    auto finite = asExtrusion(
+        evaluate("linear_extrude(height=20, scale=2) square(10);").roots[0]);
+    REQUIRE(finite.params.at("scale_x") == Approx(2.0));
+    REQUIRE(finite.params.at("scale_y") == Approx(2.0));
+}
+
 TEST_CASE("CsgEval:rotate_extrude angle keeps NaN/Infinity distinct from a literal 0",
           "[csg][v87][bugfix]") {
     // Real OpenSCAD (Value::getFiniteDouble()) treats a non-finite angle=