diff --git a/.github/workflows/single-file-build-and-test.yml b/.github/workflows/single-file-build-and-test.yml
index 1028c477..8a9dd3eb 100644
--- a/.github/workflows/single-file-build-and-test.yml
+++ b/.github/workflows/single-file-build-and-test.yml
@@ -38,7 +38,7 @@ jobs:
 
       - name: Build single-file package
         shell: cmd
-        run: python tools/bin/make-single-file --units meters seconds --version-id NA > au.hh
+        run: python tools/bin/make-single-file --units meters seconds --constants speed_of_light --version-id NA > au.hh
 
       - name: Build and run test
         shell: cmd
diff --git a/au/code/au/constant.hh b/au/code/au/constant.hh
index a84dc375..6e7ac13a 100644
--- a/au/code/au/constant.hh
+++ b/au/code/au/constant.hh
@@ -56,7 +56,8 @@ struct Constant : detail::MakesQuantityFromNumber<Constant, Unit>,
     // Convert this constant to a Quantity of the given unit and rep.
     template <typename T, typename OtherUnit>
     constexpr auto as(OtherUnit u) const {
-        static_assert(can_store_value_in<T>(u), "Cannot represent constant in this unit/rep");
+        static_assert(can_store_value_in<T>(OtherUnit{}),
+                      "Cannot represent constant in this unit/rep");
         return coerce_as<T>(u);
     }
 
@@ -69,7 +70,8 @@ struct Constant : detail::MakesQuantityFromNumber<Constant, Unit>,
     // Get the value of this constant in the given unit and rep.
     template <typename T, typename OtherUnit>
     constexpr auto in(OtherUnit u) const {
-        static_assert(can_store_value_in<T>(u), "Cannot represent constant in this unit/rep");
+        static_assert(can_store_value_in<T>(OtherUnit{}),
+                      "Cannot represent constant in this unit/rep");
         return coerce_in<T>(u);
     }
 
diff --git a/single-file-test.cc b/single-file-test.cc
index 62e8bd99..439b7552 100644
--- a/single-file-test.cc
+++ b/single-file-test.cc
@@ -26,6 +26,8 @@
 // dependencies outside of the C++14 standard library, and the single-file package of Au itself.
 
 using namespace au;
+using ::au::symbols::m;
+using ::au::symbols::s;
 
 // This ad hoc utility is a stand-in for GTEST, which we can't use here.
 template <typename ExpectedT, typename ActualT>
@@ -42,6 +44,7 @@ int main(int argc, char **argv) {
     const std::vector<bool> results{
         {
             expect_equal((meters / second)(5) * seconds(6), meters(30)),
+            expect_equal(SPEED_OF_LIGHT.as<int>(m / s), 299'792'458 * m / s),
         },
     };
     return std::all_of(std::begin(results), std::end(results), [](auto x) { return x; }) ? 0 : 1;