diff --git a/src/MNH/read_exsegn.f90 b/src/MNH/read_exsegn.f90
index 8fed638204337e21245bc816971f3e56cec76e1f..71f83b07869cb50dceb0581d5f0c5b45c3e60dd5 100644
--- a/src/MNH/read_exsegn.f90
+++ b/src/MNH/read_exsegn.f90
@@ -2994,6 +2994,14 @@ IF (CLES_NORM_TYPE=='EKMA' .AND. .NOT. LCORIO) THEN
   CLES_NORM_TYPE='NONE'
 END IF
 !
+IF (LLES_CS_MASK .AND. .NOT. LCONDSAMP) THEN
+  WRITE(ILUOUT,FMT=*) 'YOU WANT TO USE CONDITIONAL SAMPLING LES DIAG'
+  WRITE(ILUOUT,FMT=*) 'BUT THE CONDITIONAL SAMPLING IS NOT ACTIVATED'
+  WRITE(ILUOUT,FMT=*) 'USE NAM_CONDSAMP LCONDSAMP=T'
+  CALL PRINT_MSG(NVERB_FATAL,'GEN','READ_EXSEG_n','WITH LLES_CS_MASK=T YOU MUST USE LCONDSAMP=T')
+END IF
+!
+!
 !*       4.7  Check the coherence with LNUMDIFF
 !
 IF (L1D .AND. (LNUMDIFU .OR. LNUMDIFTH .OR. LNUMDIFSV) ) THEN