diff --git a/bits/27_csfutils.js b/bits/27_csfutils.js
index 8a087d7..495e5db 100644
--- a/bits/27_csfutils.js
+++ b/bits/27_csfutils.js
@@ -4,7 +4,7 @@ function fix_row(cstr/*:string*/)/*:string*/ { return cstr.replace(/([A-Z]|^)(\d
 function unfix_row(cstr/*:string*/)/*:string*/ { return cstr.replace(/\$(\d+)$/,"$1"); }
 
 function decode_col(colstr/*:string*/)/*:number*/ { var c = unfix_col(colstr), d = 0, i = 0; for(; i !== c.length; ++i) d = 26*d + c.charCodeAt(i) - 64; return d - 1; }
-function encode_col(col/*:number*/)/*:string*/ { var s=""; for(++col; col; col=Math.floor((col-1)/26)) s = String.fromCharCode(((col-1)%26) + 65) + s; return s; }
+function encode_col(col/*:number*/)/*:string*/ { if(col < 0) throw new Error("invalid column " + col); var s=""; for(++col; col; col=Math.floor((col-1)/26)) s = String.fromCharCode(((col-1)%26) + 65) + s; return s; }
 function fix_col(cstr/*:string*/)/*:string*/ { return cstr.replace(/^([A-Z])/,"$$$1"); }
 function unfix_col(cstr/*:string*/)/*:string*/ { return cstr.replace(/^\$([A-Z])/,"$1"); }