diff --git a/fortran/src/p0004.f90 b/fortran/src/p0004.f90
index 81303a11..d0b2358e 100644
--- a/fortran/src/p0004.f90
+++ b/fortran/src/p0004.f90
@@ -19,7 +19,7 @@ integer function p0004() result(answer)
                 prod = i * j
                 is_palindrome = .true.
                 write(string, '(I0)') prod
-                length = len(string)
+                length = len_trim(string)
 
                 do k = 1, length
                     n = length - k + 1