diff --git a/src/fix_press_berendsen.cpp b/src/fix_press_berendsen.cpp
index bee87f1111..31f200976c 100644
--- a/src/fix_press_berendsen.cpp
+++ b/src/fix_press_berendsen.cpp
@@ -33,6 +33,7 @@ using namespace LAMMPS_NS;
 #define MAX(A,B) ((A) > (B)) ? (A) : (B)
 
 enum{NOBIAS,BIAS};
+enum{XYZ,XY,YZ,XZ,ANISO};
 
 /* ---------------------------------------------------------------------- */
 
@@ -50,7 +51,7 @@ FixPressBerendsen::FixPressBerendsen(LAMMPS *lmp, int narg, char **arg) :
   if (strcmp(arg[3],"xyz") == 0) {
     if (narg < 7) error->all("Illegal fix press/berendsen command");
 
-    press_couple = 0;
+    press_couple = XYZ;
     p_start[0] = p_start[1] = p_start[2] = atof(arg[4]);
     p_stop[0] = p_stop[1] = p_stop[2] = atof(arg[5]);
     p_period[0] = p_period[1] = p_period[2] = atof(arg[6]);
@@ -61,16 +62,16 @@ FixPressBerendsen::FixPressBerendsen(LAMMPS *lmp, int narg, char **arg) :
     }
 
   } else {
-    if (strcmp(arg[3],"xy") == 0) press_couple = 1;
-    else if (strcmp(arg[3],"yz") == 0) press_couple = 2;
-    else if (strcmp(arg[3],"xz") == 0) press_couple = 3;
-    else if (strcmp(arg[3],"aniso") == 0) press_couple = 4;
+    if (strcmp(arg[3],"xy") == 0) press_couple = XY;
+    else if (strcmp(arg[3],"yz") == 0) press_couple = YZ;
+    else if (strcmp(arg[3],"xz") == 0) press_couple = XZ;
+    else if (strcmp(arg[3],"aniso") == 0) press_couple = ANISO;
     else error->all("Illegal fix press/berendsen command");
 
     if (narg < 8) error->all("Illegal fix press/berendsen command");
 
     if (domain->dimension == 2 && 
-	(press_couple == 1 || press_couple == 2 || press_couple == 3))
+	(press_couple == XY || press_couple == YZ || press_couple == XZ))
       error->all("Invalid fix press/berendsen command for a 2d simulation");
 
     if (strcmp(arg[4],"NULL") == 0) {
@@ -112,7 +113,7 @@ FixPressBerendsen::FixPressBerendsen(LAMMPS *lmp, int narg, char **arg) :
   allremap = 1;
 
   int iarg;
-  if (press_couple == 0) iarg = 7;
+  if (press_couple == XYZ) iarg = 7;
   else iarg = 11;
 
   while (iarg < narg) {
@@ -133,6 +134,23 @@ FixPressBerendsen::FixPressBerendsen(LAMMPS *lmp, int narg, char **arg) :
 
   // error checks
 
+  if (press_couple == XY && (p_flag[0] == 0 || p_flag[1] == 0))
+    error->all("Invalid fix npt command");
+  if (press_couple == YZ && (p_flag[1] == 0 || p_flag[2] == 0))
+    error->all("Invalid fix npt command");
+  if (press_couple == XZ && (p_flag[0] == 0 || p_flag[2] == 0))
+    error->all("Invalid fix npt command");
+
+  if (press_couple == XY && 
+      (p_start[0] != p_start[1] || p_stop[0] != p_stop[1]))
+    error->all("Invalid fix npt command");
+  if (press_couple == YZ && 
+      (p_start[1] != p_start[2] || p_stop[1] != p_stop[2]))
+    error->all("Invalid fix npt command");
+  if (press_couple == XZ && 
+      (p_start[0] != p_start[2] || p_stop[0] != p_stop[2]))
+    error->all("Invalid fix npt command");
+
   if (p_flag[0] && domain->xperiodic == 0)
     error->all("Cannot use fix press/berendsen on a non-periodic dimension");
   if (p_flag[1] && domain->yperiodic == 0)
@@ -281,7 +299,7 @@ void FixPressBerendsen::end_of_step()
 {
   // compute new T,P
 
-  if (press_couple == 0) {
+  if (press_couple == XYZ) {
     double tmp = temperature->compute_scalar();
     tmp = pressure->compute_scalar();
   } else {
@@ -319,21 +337,21 @@ void FixPressBerendsen::couple()
 {
   double *tensor = pressure->vector;
 
-  if (press_couple == 0)
+  if (press_couple == XYZ)
     p_current[0] = p_current[1] = p_current[2] = pressure->scalar;
-  else if (press_couple == 1) {
+  else if (press_couple == XY) {
     double ave = 0.5 * (tensor[0] + tensor[1]);
     p_current[0] = p_current[1] = ave;
     p_current[2] = tensor[2];
-  } else if (press_couple == 2) {
+  } else if (press_couple == YZ) {
     double ave = 0.5 * (tensor[1] + tensor[2]);
     p_current[1] = p_current[2] = ave;
     p_current[0] = tensor[0];
-  } else if (press_couple == 3) {
+  } else if (press_couple == XZ) {
     double ave = 0.5 * (tensor[0] + tensor[2]);
     p_current[0] = p_current[2] = ave;
     p_current[1] = tensor[1];
-  } if (press_couple == 4) {
+  } if (press_couple == ANISO) {
     p_current[0] = tensor[0];
     p_current[1] = tensor[1];
     p_current[2] = tensor[2];