diff --git a/src/libfsm/clone.c b/src/libfsm/clone.c
index 50f406825..001e50c18 100644
--- a/src/libfsm/clone.c
+++ b/src/libfsm/clone.c
@@ -118,7 +118,9 @@ copy_capture_actions(struct fsm *dst, const struct fsm *src)
 }
 
 struct copy_end_ids_env {
+#ifndef NDEBUG
 	char tag;
+#endif
 	struct fsm *dst;
 	const struct fsm *src;
 	bool ok;
diff --git a/src/libfsm/consolidate.c b/src/libfsm/consolidate.c
index 49b08a821..60b7c3e59 100644
--- a/src/libfsm/consolidate.c
+++ b/src/libfsm/consolidate.c
@@ -36,7 +36,9 @@ struct mapping_closure {
 };
 
 struct consolidate_copy_capture_actions_env {
+#ifndef NDEBUG
 	char tag;
+#endif
 	struct fsm *dst;
 	size_t mapping_count;
 	const fsm_state_t *mapping;
diff --git a/src/libfsm/determinise_internal.h b/src/libfsm/determinise_internal.h
index 8628f71b5..cfd4ea663 100644
--- a/src/libfsm/determinise_internal.h
+++ b/src/libfsm/determinise_internal.h
@@ -83,7 +83,9 @@ struct reverse_mapping {
 };
 
 struct det_copy_capture_actions_env {
+#ifndef NDEBUG
 	char tag;
+#endif
 	struct fsm *dst;
 	struct reverse_mapping *reverse_mappings;
 	bool ok;
diff --git a/src/libfsm/endids.c b/src/libfsm/endids.c
index 62f279ca3..f107ea058 100644
--- a/src/libfsm/endids.c
+++ b/src/libfsm/endids.c
@@ -753,7 +753,9 @@ fsm_endid_get(const struct fsm *fsm, fsm_state_t end_state,
 }
 
 struct carry_env {
+#ifndef NDEBUG
 	char tag;
+#endif
 	struct fsm *dst;
 	fsm_state_t dst_state;
 	bool ok;
diff --git a/src/libfsm/epsilons.c b/src/libfsm/epsilons.c
index dcd46ddda..35058e7f0 100644
--- a/src/libfsm/epsilons.c
+++ b/src/libfsm/epsilons.c
@@ -28,7 +28,9 @@
 #define DEF_CARRY_ENDIDS_COUNT 2
 
 struct remap_env {
+#ifndef NDEBUG
 	char tag;
+#endif
 	bool ok;
 	const struct fsm_alloc *alloc;
 	struct state_set **rmap;
diff --git a/src/libfsm/merge.c b/src/libfsm/merge.c
index b2a556c66..e75925d0a 100644
--- a/src/libfsm/merge.c
+++ b/src/libfsm/merge.c
@@ -26,7 +26,9 @@
 #define LOG_MERGE_ENDIDS 0
 
 struct copy_capture_env {
+#ifndef NDEBUG
 	char tag;
+#endif
 	bool ok;
 	struct fsm *dst;
 };