diff --git a/src/pel_stack/program_dag/program_dag.c b/src/pel_stack/program_dag/program_dag.c
index e45da67..94a5a44 100644
--- a/src/pel_stack/program_dag/program_dag.c
+++ b/src/pel_stack/program_dag/program_dag.c
@@ -169,6 +169,7 @@ typedef enum {
   AMDUAT_PEL_PREP_ERROR_INPUT_ARITY,
   AMDUAT_PEL_PREP_ERROR_PARAM_DECODE,
   AMDUAT_PEL_PREP_ERROR_CYCLE,
+  AMDUAT_PEL_PREP_ERROR_NODE_OUTPUT_INDEX,
   AMDUAT_PEL_PREP_ERROR_ROOT_MISSING,
   AMDUAT_PEL_PREP_ERROR_ROOT_OUTPUT_INDEX
 } amduat_pel_program_prep_error_t;
@@ -630,10 +631,19 @@ static amduat_pel_program_prepare_result_t amduat_program_prepare(
         }
         if (input->value.node.output_index >=
             prepared->ops[dep_index]->outputs_len) {
+          if (out_error != NULL) {
+            out_error->kind = AMDUAT_PEL_PREP_ERROR_NODE_OUTPUT_INDEX;
+            out_error->node_id = input->value.node.node_id;
+            out_error->index = input->value.node.output_index;
+          }
           amduat_prepared_free(prepared);
           return AMDUAT_PEL_PROGRAM_PREP_INVALID;
         }
       } else if (input->kind != AMDUAT_PEL_DAG_INPUT_EXTERNAL) {
+        if (out_error != NULL) {
+          out_error->kind = AMDUAT_PEL_PREP_ERROR_INVALID_INPUT_KIND;
+          out_error->node_id = node->id;
+        }
         amduat_prepared_free(prepared);
         return AMDUAT_PEL_PROGRAM_PREP_INVALID;
       }
@@ -806,6 +816,13 @@ static bool amduat_pel_program_dag_exec_internal(
                          AMDUAT_PEL_DAG_DIAG_CYCLE,
                          "invalid program: dependency cycle");
         break;
+      case AMDUAT_PEL_PREP_ERROR_NODE_OUTPUT_INDEX:
+        amduat_diag_setf(out_result,
+                         AMDUAT_PEL_DAG_DIAG_OUTPUT_INDEX,
+                         "invalid program: node %u output index %u out of range",
+                         (unsigned int)prep_error.node_id,
+                         (unsigned int)prep_error.index);
+        break;
       case AMDUAT_PEL_PREP_ERROR_ROOT_MISSING:
         amduat_diag_setf(out_result,
                          AMDUAT_PEL_DAG_DIAG_MISSING_NODE,