diff --git a/src/FixFunctionArgument.jl b/src/FixFunctionArgument.jl
index 449f229..cc0d185 100644
--- a/src/FixFunctionArgument.jl
+++ b/src/FixFunctionArgument.jl
@@ -17,10 +17,17 @@ module FixFunctionArgument
     @noinline function throw_too_few_args()
         throw(ArgumentError("too few positional arguments given in call of a `Fix`"))
     end
+    @noinline function throw_not_int()
+        throw(ArgumentError("the type parameter `N` must be an `Int`"))
+    end
     @noinline function throw_not_positive()
         throw(ArgumentError("the type parameter `N` must be greater than zero"))
     end
-    @inline function check_positive(n::Int)
+    @inline function check_positive(n)
+        if !(n isa Int)
+            throw_not_int()
+        end
+        n = n::Int
         if n < 1
             throw_not_positive()
         end
diff --git a/test/runtests.jl b/test/runtests.jl
index 57d1c9c..fc2f0ae 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -13,6 +13,16 @@ using Test
     end
     @testset "basic" begin
         @test sin(0.3) === @inferred Fix1(sin, 0.3)()
+        @test (7 - 3) === @inferred Fix1(-, 7)(3)
+        @test (7 - 3) === @inferred Fix2(-, 3)(7)
+    end
+    @testset "invalid `N`" begin
+        for N ∈ (-1, 0, true)
+            @test_throws ArgumentError Fix{N}(+, 7)
+        end
+    end
+    @testset "too few arguments in call" begin
+        @test_throws ArgumentError Fix{10}(Returns(nothing), 7)(1, 2, 3)
     end
 end