diff --git a/bin/env.sh b/bin/env.sh
index b4d0d2bb8..51e8f1ca4 100644
--- a/bin/env.sh
+++ b/bin/env.sh
@@ -4,13 +4,13 @@ thisEnv=${BASH_SOURCE[0]:-$0}
 export CLAS12DIR=$(realpath $(dirname $thisEnv)/..)
 
 # Set default field maps (but do not override user's env):
-if [ -z "$COAT_MAGFIELD_TORUSMAP" ]; then
+if [ -z "${COAT_MAGFIELD_TORUSMAP-}" ]; then
     export COAT_MAGFIELD_TORUSMAP=Symm_torus_r2501_phi16_z251_24Apr2018.dat
 fi
-if [ -z "$COAT_MAGFIELD_TORUSSECONDARYMAP" ]; then
+if [ -z "${COAT_MAGFIELD_TORUSSECONDARYMAP-}" ]; then
     export COAT_MAGFIELD_TORUSSECONDARYMAP=Full_torus_r251_phi181_z251_25Jan2021.dat
 fi
-if [ -z "$COAT_MAGFIELD_SOLENOIDMAP" ]; then
+if [ -z "${COAT_MAGFIELD_SOLENOIDMAP-}" ]; then
     export COAT_MAGFIELD_SOLENOIDMAP=Symm_solenoid_r601_phi1_z1201_13June2018.dat
 fi