diff --git a/src/libslic3r/Print.cpp b/src/libslic3r/Print.cpp
index 3cd42eb96e..e474818dfe 100644
--- a/src/libslic3r/Print.cpp
+++ b/src/libslic3r/Print.cpp
@@ -566,7 +566,7 @@ std::vector<unsigned int> Print::extruders(bool conside_custom_gcode) const
 
     // If a wipe tower filament is explicitly set, ensure it participates in tool ordering.
     if (has_wipe_tower() && config().wipe_tower_filament != 0 && extruders.size() > 1) {
-        assert(config().wipe_tower_filament > 0 && config().wipe_tower_filament < int(config().nozzle_diameter.size()));
+        assert(config().wipe_tower_filament > 0 && config().wipe_tower_filament <= int(config().filament_diameter.size()));
         extruders.emplace_back(config().wipe_tower_filament - 1); // config value is 1-based
     }
 
@@ -1472,6 +1472,17 @@ StringObjectException Print::validate(std::vector<StringObjectException> *warnin
         }
 
     if (this->has_wipe_tower() && ! m_objects.empty()) {
+        // Orca: wipe_tower_filament (issue #10971) is inserted into the tool order after
+        // resolve_mixed_filaments has expanded every mixed (virtual) slot, so a mixed slot here
+        // would reach the G-code as a tool change to a slot no nozzle carries. The GUI hides
+        // mixed slots from the option; this guards loaded projects and the CLI.
+        if (m_config.wipe_tower_filament > 0) {
+            const auto  &is_mixed = m_config.filament_is_mixed.values;
+            const size_t wipe_idx = size_t(m_config.wipe_tower_filament - 1);
+            if (wipe_idx < is_mixed.size() && is_mixed[wipe_idx])
+                return { L("The wipe tower filament cannot be a mixed filament."), nullptr, "wipe_tower_filament" };
+        }
+
         // Make sure all extruders use same diameter filament and have the same nozzle diameter
         // EPSILON comparison is used for nozzles and 10 % tolerance is used for filaments
         double first_nozzle_diam = m_config.nozzle_diameter.get_at(extruders.front());
diff --git a/tests/fff_print/test_mixed_filament.cpp b/tests/fff_print/test_mixed_filament.cpp
index 84a3270605..143ecdcb6e 100644
--- a/tests/fff_print/test_mixed_filament.cpp
+++ b/tests/fff_print/test_mixed_filament.cpp
@@ -211,3 +211,46 @@ TEST_CASE("By-object G-code lists a mixed slot's components in the filament head
     CHECK(gc.find("; filament: 1,2\n") != std::string::npos);
     CHECK(gc.find("; filament: 3") == std::string::npos);
 }
+
+TEST_CASE("Print::validate rejects a mixed filament as the wipe tower filament", "[MixedFilament]")
+{
+    // The validate backstop refuses a mixed (virtual) slot as the wipe tower filament; the GUI hides
+    // the slot from that option. Two cubes on physical filaments 1 and 2 make the tower real, and the
+    // region roles mixed_config() points at the slot are reset so only the tower uses it.
+    DynamicPrintConfig config = mixed_config(false);
+    config.set_deserialize_strict({
+        {"enable_prime_tower",         "1"},
+        {"wipe_tower_x",               "50"}, // inside the 200x200 test bed
+        {"wipe_tower_y",               "50"}, // (the default y, 220, is not)
+        {"layer_change_gcode",         "G92 E0\n"}, // validate() relative-E reset, as in test_print.cpp's build_cubes
+        {"outer_wall_filament_id",     "0"},
+        {"inner_wall_filament_id",     "0"},
+        {"sparse_infill_filament_id",  "0"},
+        {"internal_solid_filament_id", "0"},
+        {"top_surface_filament_id",    "0"},
+        {"bottom_surface_filament_id", "0"},
+    });
+    const std::vector<std::vector<ConfigBase::SetDeserializeItem>> overrides{ { {"extruder", "1"} }, { {"extruder", "2"} } };
+
+    SECTION("a physical wipe tower filament validates") {
+        config.set_deserialize_strict({{"wipe_tower_filament", "2"}});
+        Print print;
+        Model model;
+        init_print(std::vector<TriangleMesh>{cube(20), cube(20)}, print, model, config, &overrides);
+        REQUIRE(print.has_wipe_tower());
+        const StringObjectException err = print.validate();
+        INFO(err.string);
+        CHECK(err.string.empty());
+    }
+
+    SECTION("the mixed slot is refused") {
+        config.set_deserialize_strict({{"wipe_tower_filament", "3"}});
+        Print print;
+        Model model;
+        init_print(std::vector<TriangleMesh>{cube(20), cube(20)}, print, model, config, &overrides);
+        REQUIRE(print.has_wipe_tower());
+        const StringObjectException err = print.validate();
+        CHECK_FALSE(err.string.empty());
+        CHECK(err.opt_key == "wipe_tower_filament");
+    }
+}