diff --git a/src/variable.cpp b/src/variable.cpp
index 35eb0781b2..efdc701c1a 100644
--- a/src/variable.cpp
+++ b/src/variable.cpp
@@ -4057,7 +4057,7 @@ int Variable::special_function(char *word, char *contents, Tree **tree,
                                Tree **treestack, int &ntreestack,
                                double *argstack, int &nargstack, int ivar)
 {
-  bigint sx,sxx;
+  double sx,sxx;
   double value,sy,sxy;
 
   // word not a match to any special function
@@ -4213,10 +4213,7 @@ int Variable::special_function(char *word, char *contents, Tree **tree,
                            "variable formula",ivar);
 
     value = 0.0;
-    if (method == SLOPE) {
-      sx = sxx = 0;
-      sy = sxy = 0.0;
-    }
+    if (method == SLOPE) sx = sxx = sy = sxy = 0.0;
     else if (method == XMIN) value = BIG;
     else if (method == XMAX) value = -BIG;
 
@@ -4235,10 +4232,10 @@ int Variable::special_function(char *word, char *contents, Tree **tree,
         else if (method == AVE) value += vec[j];
         else if (method == TRAP) value += vec[j];
         else if (method == SLOPE) {
-          sx += i;
+          sx += (double)i;
           sy += vec[j];
-          sxx += i*i;
-          sxy += i*vec[j];
+          sxx += (double)i * (double)i;
+          sxy += (double)i * vec[j];
         }
         j += nstride;
       }
@@ -4256,10 +4253,10 @@ int Variable::special_function(char *word, char *contents, Tree **tree,
         else if (method == AVE) value += one;
         else if (method == TRAP) value += one;
         else if (method == SLOPE) {
-          sx += i;
+          sx += (double)i;
           sy += one;
-          sxx += i*i;
-          sxy += i*one;
+          sxx += (double)i * (double)i;
+          sxy += (double)i * one;
         }
       }
       if (method == TRAP) {
@@ -4281,10 +4278,10 @@ int Variable::special_function(char *word, char *contents, Tree **tree,
         else if (method == AVE) value += one;
         else if (method == TRAP) value += one;
         else if (method == SLOPE) {
-          sx += i;
+          sx += (double) i;
           sy += one;
-          sxx += i*i;
-          sxy += i*one;
+          sxx += (double)i * (double)i;
+          sxy += (double)i * one;
         }
       }
       if (method == TRAP) value -= 0.5*vec[0] + 0.5*vec[nvec-1];