diff --git a/compare_dg_tests.pl b/compare_dg_tests.pl
index cc45605..0c49bd5 100755
--- a/compare_dg_tests.pl
+++ b/compare_dg_tests.pl
@@ -416,25 +416,42 @@
    my $rtotal = 0;
    my $quiet_reg = $quiet;
    my $ref_ratio = 0;
+   my $ref_total = 0;
    my $res_ratio = 0;
+   my $res_total = 0;
    my $ratio_thresh = 0.95; # Warn if pass ratio is below this threshold
    my $ignore_exec = 0;
    my $ref_has_exec_tests = 0;
    my $res_has_exec_tests = 0;
 
    # Compute the pass ratio as a sanity check
-   $ref_ratio = ($ref->{PASS} + $ref->{XFAIL}) /
-       ($ref->{PASS} + $ref->{XFAIL}
-	+ $ref->{XPASS} + $ref->{FAIL}
-	+ $ref->{UNRESOLVED}
-	+ $ref->{UNSUPPORTED}
-	+ $ref->{UNTESTED});
-   $res_ratio = ($res->{PASS} + $res->{XFAIL}) /
-       ($res->{PASS} + $res->{XFAIL}
-	+ $res->{XPASS} + $res->{FAIL}
-	+ $res->{UNRESOLVED}
-	+ $res->{UNSUPPORTED}
-	+ $res->{UNTESTED});
+   $ref_total = $ref->{PASS} +
+       $ref->{XFAIL} +
+       $ref->{XPASS} +
+       $ref->{FAIL} +
+       $ref->{UNRESOLVED} +
+       $ref->{UNSUPPORTED} +
+       $ref->{UNTESTED};
+
+   # It is possible that no test is executed at all (for instance if
+   # RUNTESTFLAGS was too restrictive). Avoid division by 0.
+   if ($ref_total > 0) {
+       $ref_ratio = ($ref->{PASS} + $ref->{XFAIL}) /
+	   $ref_total;
+   }
+
+   $res_total = $res->{PASS} +
+       $res->{XFAIL} +
+       $res->{XPASS} +
+       $res->{FAIL} +
+       $res->{UNRESOLVED} +
+       $res->{UNSUPPORTED} +
+       $res->{UNTESTED};
+
+   if ($res_total > 0) {
+       $res_ratio = ($res->{PASS} + $res->{XFAIL}) /
+	   $res_total;
+   }
 
    if (not $quiet)
    {