diff --git a/libopenage/renderer/opengl/shader_program.cpp b/libopenage/renderer/opengl/shader_program.cpp
index 395ee395c2..4b46a8263c 100644
--- a/libopenage/renderer/opengl/shader_program.cpp
+++ b/libopenage/renderer/opengl/shader_program.cpp
@@ -424,8 +424,11 @@ void GlShaderProgram::update_uniforms(std::shared_ptr<GlUniformInput> const &uni
 			glBindTexture(GL_TEXTURE_2D, tex);
 			// TODO: maybe call this at a more appropriate position
 			glUniform1i(loc, tex_unit_id);
-			auto &tex_value = *this->textures_per_texunits[tex_unit_id];
-			tex_value = tex;
+			ENSURE(tex_unit_id < this->textures_per_texunits.size(),
+			       "Tried to assign texture to non-existant texture unit at index "
+			           << tex_unit_id
+			           << " (max: " << this->textures_per_texunits.size() << ").");
+			this->textures_per_texunits[tex_unit_id] = tex;
 			break;
 		}
 		default:
diff --git a/libopenage/renderer/opengl/shader_program.h b/libopenage/renderer/opengl/shader_program.h
index 5d31f2a071..e5c75374c3 100644
--- a/libopenage/renderer/opengl/shader_program.h
+++ b/libopenage/renderer/opengl/shader_program.h
@@ -195,6 +195,7 @@ class GlShaderProgram final : public ShaderProgram
 	std::unordered_map<std::string, GlVertexAttrib> attribs;
 
 	/// Store which texture handles are currently bound to the shader's texture units.
+	/// A value of std::nullopt means the texture unit is unbound (no texture assigned).
 	std::vector<std::optional<GLuint>> textures_per_texunits;
 
 	/// Whether this program has been validated.