diff --git a/vector.h b/vector.h
index ef9504e..b82a7cc 100644
--- a/vector.h
+++ b/vector.h
@@ -93,6 +93,13 @@ SOFTWARE.
 
 typedef void (*vector_destructor_t)(void* element); // Function pointer type for element destructor
 
+/*
+ A vector. Do not construct, copy or modify this struct directly - the fields are
+ visible only because the functions below are inline. Create vectors with
+ vector_create() and use the accessors. A vector_t that did not come from
+ vector_create() is not a valid vector, and the functions will reject it where
+ they can and misbehave where they cannot.
+*/
 typedef struct {
     size_t size; // Number of elements in the vector
     size_t capacity; // Allocated capacity of the vector
@@ -297,6 +304,10 @@ static inline int vector_prune(vector_t* vec) {
         return -1; // Invalid vector
     }
 
+    if (vec->element_size == 0) {
+        return -1; // Not a usable vector; also guards the division below
+    }
+
     if (vec->size < vec->capacity) {
         size_t new_capacity = vec->size > 0 ? vec->size : 1; // Ensure capacity is at least 1