diff --git a/base/strings/substring.jl b/base/strings/substring.jl
index 792925f24b12b..176c42cccf9f7 100644
--- a/base/strings/substring.jl
+++ b/base/strings/substring.jl
@@ -226,7 +226,7 @@ end
 
 string(a::Union{Char, String, SubString{String}, Symbol}...) = _string(a...)
 
-function _string(a::Union{Char, String, SubString{String}, Symbol}...)
+function _string(a...)
     n = 0
     for v in a
         # 4 types is too many for automatic Union-splitting, so we split manually