diff --git a/lib/pure/base64.nim b/lib/pure/base64.nim
index 6af5345f29..591d22cc00 100644
--- a/lib/pure/base64.nim
+++ b/lib/pure/base64.nim
@@ -244,7 +244,7 @@ proc decode*(s: string): string =
     inputLen = s.len
     inputEnds = 0
   # strip trailing characters
-  while s[inputLen - 1] in {'\n', '\r', ' ', '='}:
+  while inputLen > 0 and s[inputLen - 1] in {'\n', '\r', ' ', '='}:
     dec inputLen
   # hot loop: read 4 characters at at time
   inputEnds = inputLen - 4
diff --git a/tests/stdlib/tbase64.nim b/tests/stdlib/tbase64.nim
index 98388bb6c5..c3bfb818e2 100644
--- a/tests/stdlib/tbase64.nim
+++ b/tests/stdlib/tbase64.nim
@@ -18,6 +18,8 @@ template main() =
   doAssert encode("") == ""
   doAssert decode("") == ""
 
+  doAssert decode(" ") == ""
+
   const testInputExpandsTo76 = "+++++++++++++++++++++++++++++++++++++++++++++++++++++++++"
   const testInputExpands = "++++++++++++++++++++++++++++++"
   const longText = """Man is distinguished, not only by his reason, but by this