diff --git a/src/sys.c b/src/sys.c
index d426e4cbc9a73..3295cc327a4dd 100644
--- a/src/sys.c
+++ b/src/sys.c
@@ -631,9 +631,9 @@ JL_DLLEXPORT long jl_SC_CLK_TCK(void)
 
 JL_DLLEXPORT size_t jl_get_field_offset(jl_datatype_t *ty, int field)
 {
-    if (field > jl_datatype_nfields(ty))
+    if (field > jl_datatype_nfields(ty) || field < 1)
         jl_error("This type does not have that many fields");
-    return jl_field_offset(ty, field);
+    return jl_field_offset(ty, field-1);
 }
 
 JL_DLLEXPORT size_t jl_get_alignment(jl_datatype_t *ty)