diff --git a/base/replutil.jl b/base/replutil.jl
index bcd52f5e886c2..609b81609b984 100644
--- a/base/replutil.jl
+++ b/base/replutil.jl
@@ -13,6 +13,10 @@ function writemime(io::IO, ::MIME"text/plain", f::Function)
     print(io, ns, " (", what, " with $n $m)")
 end
 
+function writemime(io::IO, ::MIME"text/plain", f::Builtin)
+    print(io, typeof(f).name.mt.name, " (built-in function)")
+end
+
 # writemime for ranges, e.g.
 #  3-element UnitRange{Int64,Int}
 #   1,2,3
diff --git a/src/toplevel.c b/src/toplevel.c
index 057e51c1d37ab..6f19ff2d673a1 100644
--- a/src/toplevel.c
+++ b/src/toplevel.c
@@ -781,6 +781,9 @@ JL_DLLEXPORT void jl_method_def(jl_svec_t *argdata, jl_lambda_info_t *f, jl_valu
     mt = ((jl_datatype_t*)ftype)->name->mt;
     name = mt->name;
 
+    if (jl_subtype(ftype, jl_builtin_type, 0))
+        jl_error("cannot add methods to a builtin function");
+
     jl_check_static_parameter_conflicts(f, tvars, name);
 
     // TODO