diff --git a/base/c.jl b/base/c.jl
index b79c6a4112387..87238f4b6580c 100644
--- a/base/c.jl
+++ b/base/c.jl
@@ -67,7 +67,7 @@ containsnul(p::Ptr, len) = C_NULL != ccall(:memchr, Ptr{Cchar}, (Ptr{Cchar}, Cin
 function unsafe_convert(::Type{Cstring}, s::ByteString)
     p = unsafe_convert(Ptr{Cchar}, s)
     if containsnul(p, sizeof(s))
-        throw(ArgumentError("embedded NUL chars are not allowed in C strings"))
+        throw(ArgumentError("embedded NUL chars are not allowed in C strings: $(repr(s))"))
     end
     return Cstring(p)
 end
diff --git a/base/string.jl b/base/string.jl
index ebe3c8db8fe1e..baad4586acdd8 100644
--- a/base/string.jl
+++ b/base/string.jl
@@ -1722,7 +1722,7 @@ wstring(s::Cwstring) = wstring(box(Ptr{Cwchar_t}, unbox(Cwstring,s)))
 # to have WString
 function unsafe_convert(::Type{Cwstring}, s::WString)
     if containsnul(s)
-        throw(ArgumentError("embedded NUL chars are not allowed in C strings"))
+        throw(ArgumentError("embedded NUL chars are not allowed in C strings: $(repr(s))"))
     end
     return Cwstring(unsafe_convert(Ptr{Cwchar_t}, s))
 end