diff --git a/apps/web/src/components/viewers/cardMaterial.test.ts b/apps/web/src/components/viewers/cardMaterial.test.ts
index be71c51..ae5de3e 100644
--- a/apps/web/src/components/viewers/cardMaterial.test.ts
+++ b/apps/web/src/components/viewers/cardMaterial.test.ts
@@ -18,8 +18,11 @@ describe('applyMapTransform', () => {
     // Uniforms are copied, so the caller's vectors stay reusable.
     expect(shader.uniforms.uMapRepeat!.value).toEqual(new THREE.Vector2(0.5, 0.25));
     expect(shader.uniforms.uMapOffset!.value).toEqual(new THREE.Vector2(0.1, 0.2));
-    // The override is injected right after the chunk include, which stays in
-    // place (it declares `vMapUv`/`uv`).
+    // The uniforms are declared in the GLSL (three.js does not auto-declare
+    // uniforms added via `onBeforeCompile`), and the override is injected right
+    // after the chunk include, which stays in place (it declares `vMapUv`/`uv`).
+    expect(shader.vertexShader).toContain('uniform vec2 uMapRepeat;');
+    expect(shader.vertexShader).toContain('uniform vec2 uMapOffset;');
     expect(shader.vertexShader).toContain('#include <uv_vertex>\n\tvMapUv = uv * uMapRepeat + uMapOffset;');
   });
 
diff --git a/apps/web/src/components/viewers/cardMaterial.ts b/apps/web/src/components/viewers/cardMaterial.ts
index d2af5fd..9d82e95 100644
--- a/apps/web/src/components/viewers/cardMaterial.ts
+++ b/apps/web/src/components/viewers/cardMaterial.ts
@@ -14,6 +14,14 @@ import * as THREE from 'three';
  * refreshed from `map.matrix` every frame, overwriting any per-material
  * transform we set on the shared texture.
  */
+// Uniforms added via `onBeforeCompile` are not auto-declared by three.js, so
+// they must be declared in the GLSL explicitly (the built-in `mapTransform` is
+// declared in `uv_pars_vertex.glsl.js`).
+const UNIFORM_DECLS = /* glsl */ `
+	uniform vec2 uMapRepeat;
+	uniform vec2 uMapOffset;
+`;
+
 const VERTEX_INJECT = /* glsl */ `
 	#include <uv_vertex>
 	vMapUv = uv * uMapRepeat + uMapOffset;
@@ -36,9 +44,8 @@ export function applyMapTransform(
   material.onBeforeCompile = (shader) => {
     shader.uniforms.uMapRepeat = { value: r };
     shader.uniforms.uMapOffset = { value: o };
-    shader.vertexShader = shader.vertexShader.replace(
-      '#include <uv_vertex>',
-      VERTEX_INJECT,
-    );
+    shader.vertexShader =
+      UNIFORM_DECLS +
+      shader.vertexShader.replace('#include <uv_vertex>', VERTEX_INJECT);
   };
 }
\ No newline at end of file