diff --git a/lume.lua b/lume.lua
index a0b75fc..73c2b33 100644
--- a/lume.lua
+++ b/lume.lua
@@ -235,6 +235,7 @@ end
 
 
 function lume.fn(fn, ...)
+  assert(isfunction(fn), "expected a function as the first argument")
   local args = {...}
   return function(...)
     local a = lume.merge(lume.clone(args), {...})
diff --git a/test/test_lume.lua b/test/test_lume.lua
index ee1e191..57ac598 100644
--- a/test/test_lume.lua
+++ b/test/test_lume.lua
@@ -258,6 +258,7 @@ end
 tests["lume.fn"] = function()
   local f = lume.fn(function(a, b) return a + b end, 10)
   testeq( f(5),  15 )
+  tester.test.error( lume.fn, 123 )
 end
 
 -- lume.once
@@ -265,6 +266,7 @@ tests["lume.once"] = function()
   local f = lume.once(function(a, b) return a + b end, 10)
   testeq( f(5),  15   )
   testeq( f(5),  nil  )
+  tester.test.error( lume.once, 123 )
 end
 
 -- lume.time