diff --git a/planner.c b/planner.c
index 9dbe363..f21ced6 100644
--- a/planner.c
+++ b/planner.c
@@ -232,16 +232,18 @@ void planner_reverse_pass() {
 // The kernel called by planner_recalculate() when scanning the plan from first to last entry.
 void planner_forward_pass_kernel(block_t *previous, block_t *current, block_t *next) {
   if(!current) { return; }
-  // If the previous block is an acceleration block, but it is not long enough to 
-  // complete the full speed change within the block, we need to adjust out entry
-  // speed accordingly. Remember current->entry_factor equals the exit factor of 
-  // the previous block.
-  if(previous->entry_factor < current->entry_factor) {
-    double max_entry_speed = max_allowable_speed(-settings.acceleration,
-      current->nominal_speed*previous->entry_factor, previous->millimeters);
-    double max_entry_factor = max_entry_speed/current->nominal_speed;
-    if (max_entry_factor < current->entry_factor) {
-      current->entry_factor = max_entry_factor;
+  if(previous) {
+    // If the previous block is an acceleration block, but it is not long enough to 
+    // complete the full speed change within the block, we need to adjust out entry
+    // speed accordingly. Remember current->entry_factor equals the exit factor of 
+    // the previous block.
+    if(previous->entry_factor < current->entry_factor) {
+      double max_entry_speed = max_allowable_speed(-settings.acceleration,
+        current->nominal_speed*previous->entry_factor, previous->millimeters);
+      double max_entry_factor = max_entry_speed/current->nominal_speed;
+      if (max_entry_factor < current->entry_factor) {
+        current->entry_factor = max_entry_factor;
+      }
     }
   }
 }