diff -urN ../exim-4.84.orig/src/mime.c ./src/mime.c
--- ../exim-4.84.orig/src/mime.c	2014-08-09 15:44:29.000000000 +0300
+++ ./src/mime.c	2014-11-11 13:47:29.000000000 +0200
@@ -620,6 +620,7 @@
 		else
 		  param_value = string_cat(param_value, &size, &ptr, q++, 1);
 		}
+	    if (size > 0) {
 	      param_value[ptr++] = '\0';
 	      param_value_len = ptr;
 
@@ -629,6 +630,7 @@
 	      p += (mp->namelen + param_value_len + 1);
 	      goto NEXT_PARAM_SEARCH;
 	    }
+	    }
 	  }
 	  /* There is something, but not one of our interesting parameters.
 	     Advance to the next semicolon */