diff --git a/toolchain/mfc/case_validator.py b/toolchain/mfc/case_validator.py
index 6fdfc408db..f90ef0efbc 100644
--- a/toolchain/mfc/case_validator.py
+++ b/toolchain/mfc/case_validator.py
@@ -668,6 +668,10 @@ def check_riemann_solver(self):
         low_Mach = self.get("low_Mach", 0)
         cyl_coord = self.get("cyl_coord", "F") == "T"
         viscous = self.get("viscous", "F") == "T"
+        igr = self.get("igr", "F") == "T"
+
+        if igr:
+            return
 
         self.prohibit(riemann_solver is None, "riemann_solver must be specified (1=HLL, 2=HLLC, 4=HLLD, 5=Lax-Friedrichs)")
         if riemann_solver is None: