diff --git a/src/subs.jl b/src/subs.jl
index 7bdac8e..03501d5 100644
--- a/src/subs.jl
+++ b/src/subs.jl
@@ -138,7 +138,7 @@ end
 
 function _mono_eval(z::Vector{Int}, vals::AbstractVector)
     if length(z) != length(vals)
-        error("")
+        error("Cannot evaluate a polynomial of `$(length(z))` variables with only `$(length(vals))` values.")
     end
     if isempty(z)
         return one(eltype(vals))^1
diff --git a/test/runtests.jl b/test/runtests.jl
index c9f1a49..ac0c6e0 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -18,6 +18,8 @@ end
     alloc_test_lt(() -> p(x => v), 300)
     alloc_test_lt(() -> substitute(Eval(), p, v), 0)
     alloc_test_lt(() -> p(v), 0)
+    err = ErrorException("Cannot evaluate a polynomial of `3` variables with only `2` values.")
+    @test_throws err p([1, 2])
 end
 
 @testset "Issue #70" begin