diff --git a/src/expression_diff.cpp b/src/expression_diff.cpp
index f5f5f18e5..e96f519b6 100644
--- a/src/expression_diff.cpp
+++ b/src/expression_diff.cpp
@@ -1468,14 +1468,23 @@ std::vector<expression> dtens::get_jacobian() const
 
 std::uint32_t dtens::get_nvars() const
 {
-    if (p_impl->m_map.empty()) {
-        return 0;
-    }
-
     // NOTE: we ensure in the diff_tensors() implementation
     // that the number of diff variables is representable
     // by std::uint32_t.
-    return static_cast<std::uint32_t>(begin()->first.size() - 1u);
+    auto ret = static_cast<std::uint32_t>(get_args().size());
+
+#if !defined(NDEBUG)
+
+    if (p_impl->m_map.empty()) {
+        assert(ret == 0u);
+    } else {
+        assert(!begin()->first.empty());
+        assert(ret == begin()->first.size() - 1u);
+    }
+
+#endif
+
+    return ret;
 }
 
 std::uint32_t dtens::get_nouts() const
diff --git a/test/expression_diff_tensors.cpp b/test/expression_diff_tensors.cpp
index b50592fa2..aa04923c4 100644
--- a/test/expression_diff_tensors.cpp
+++ b/test/expression_diff_tensors.cpp
@@ -109,6 +109,7 @@ TEST_CASE("diff_tensors basic")
     };
 
     REQUIRE(dt.size() == 2u);
+    REQUIRE(dt.get_nvars() == 1u);
 
     assign_sr(dt.get_derivatives(0, 0));
     REQUIRE(diff_vec.size() == 1u);
@@ -127,6 +128,7 @@ TEST_CASE("diff_tensors basic")
         Message(fmt::format("Cannot locate the derivative corresponding to the indices vector {}", std::vector{0, 2})));
 
     dt = diff_tensors({1_dbl}, kw::diff_order = 2, kw::diff_args = {par[0]});
+    REQUIRE(dt.get_nvars() == 1u);
     REQUIRE(dt.size() == 3u);
     assign_sr(dt.get_derivatives(0, 0));
     REQUIRE(diff_vec.size() == 1u);
@@ -149,6 +151,7 @@ TEST_CASE("diff_tensors basic")
         Message(fmt::format("Cannot locate the derivative corresponding to the indices vector {}", std::vector{0, 3})));
 
     dt = diff_tensors({1_dbl}, kw::diff_order = 3, kw::diff_args = {par[0]});
+    REQUIRE(dt.get_nvars() == 1u);
     REQUIRE(dt.size() == 4u);
     assign_sr(dt.get_derivatives(0, 0));
     REQUIRE(diff_vec.size() == 1u);
@@ -176,6 +179,7 @@ TEST_CASE("diff_tensors basic")
 
     // Automatically deduced diff variables.
     dt = diff_tensors({x + y, x * y * y}, kw::diff_order = 2);
+    REQUIRE(dt.get_nvars() == 2u);
     REQUIRE(dt.size() == 12u);
     assign_sr(dt.get_derivatives(0));
     REQUIRE(diff_vec == std::vector{x + y, x * y * y});
@@ -186,6 +190,7 @@ TEST_CASE("diff_tensors basic")
 
     // Diff wrt all variables.
     dt = diff_tensors({x + y, x * y * y}, kw::diff_order = 2, kw::diff_args = diff_args::vars);
+    REQUIRE(dt.get_nvars() == 2u);
     REQUIRE(dt.size() == 12u);
     assign_sr(dt.get_derivatives(0));
     REQUIRE(diff_vec == std::vector{x + y, x * y * y});
@@ -196,6 +201,7 @@ TEST_CASE("diff_tensors basic")
 
     // Diff wrt some variables.
     dt = diff_tensors({x + y, x * y * y}, kw::diff_order = 2, kw::diff_args = {x});
+    REQUIRE(dt.get_nvars() == 1u);
     REQUIRE(dt.size() == 6u);
     assign_sr(dt.get_derivatives(0));
     REQUIRE(diff_vec == std::vector{x + y, x * y * y});
@@ -206,6 +212,7 @@ TEST_CASE("diff_tensors basic")
 
     // Diff wrt all params.
     dt = diff_tensors({par[0] + y, x * y * par[1]}, kw::diff_order = 2, kw::diff_args = diff_args::params);
+    REQUIRE(dt.get_nvars() == 2u);
     REQUIRE(dt.size() == 12u);
     assign_sr(dt.get_derivatives(0));
     REQUIRE(diff_vec == std::vector{par[0] + y, x * y * par[1]});
@@ -216,6 +223,7 @@ TEST_CASE("diff_tensors basic")
 
     // Diff wrt some param.
     dt = diff_tensors({par[0] + y, x * y * par[1]}, kw::diff_order = 2, kw::diff_args = {par[1]});
+    REQUIRE(dt.get_nvars() == 1u);
     REQUIRE(dt.size() == 6u);
     assign_sr(dt.get_derivatives(0));
     REQUIRE(diff_vec == std::vector{par[0] + y, x * y * par[1]});