diff --git a/lucene/core/src/java/org/apache/lucene/util/automaton/Automata.java b/lucene/core/src/java/org/apache/lucene/util/automaton/Automata.java
index 89522781fdbd..9ecf748418f6 100644
--- a/lucene/core/src/java/org/apache/lucene/util/automaton/Automata.java
+++ b/lucene/core/src/java/org/apache/lucene/util/automaton/Automata.java
@@ -43,8 +43,8 @@
  */
 public final class Automata {
   /**
-   * {@link #makeStringUnion(Collection)} limits terms of this max length to ensure the stack
-   * doesn't overflow while building, since our algorithm currently relies on recursion.
+   * {@link #makeStringUnion(Iterable)} limits terms of this max length to ensure the stack doesn't
+   * overflow while building, since our algorithm currently relies on recursion.
    */
   public static final int MAX_STRING_UNION_TERM_LENGTH = 1000;
 
@@ -576,8 +576,8 @@ public static Automaton makeString(int[] word, int offset, int length) {
    * @return An {@link Automaton} accepting all input strings. The resulting automaton is codepoint
    *     based (full unicode codepoints on transitions).
    */
-  public static Automaton makeStringUnion(Collection<BytesRef> utf8Strings) {
-    if (utf8Strings.isEmpty()) {
+  public static Automaton makeStringUnion(Iterable<BytesRef> utf8Strings) {
+    if (utf8Strings.iterator().hasNext() == false) {
       return makeEmpty();
     } else {
       return StringsToAutomaton.build(utf8Strings, false);
@@ -593,8 +593,8 @@ public static Automaton makeStringUnion(Collection<BytesRef> utf8Strings) {
    * @return An {@link Automaton} accepting all input strings. The resulting automaton is binary
    *     based (UTF-8 encoded byte transition labels).
    */
-  public static Automaton makeBinaryStringUnion(Collection<BytesRef> utf8Strings) {
-    if (utf8Strings.isEmpty()) {
+  public static Automaton makeBinaryStringUnion(Iterable<BytesRef> utf8Strings) {
+    if (utf8Strings.iterator().hasNext() == false) {
       return makeEmpty();
     } else {
       return StringsToAutomaton.build(utf8Strings, true);
diff --git a/lucene/core/src/java/org/apache/lucene/util/automaton/StringsToAutomaton.java b/lucene/core/src/java/org/apache/lucene/util/automaton/StringsToAutomaton.java
index 6c66dc6fd9ed..87b0b6ef8f0f 100644
--- a/lucene/core/src/java/org/apache/lucene/util/automaton/StringsToAutomaton.java
+++ b/lucene/core/src/java/org/apache/lucene/util/automaton/StringsToAutomaton.java
@@ -18,7 +18,6 @@
 
 import java.io.IOException;
 import java.util.Arrays;
-import java.util.Collection;
 import java.util.HashMap;
 import java.util.IdentityHashMap;
 import org.apache.lucene.util.ArrayUtil;
@@ -35,8 +34,8 @@
  * to directly build a binary {@link Automaton} representation. Users should access this
  * functionality through {@link Automata} static methods.
  *
- * @see Automata#makeStringUnion(Collection)
- * @see Automata#makeBinaryStringUnion(Collection)
+ * @see Automata#makeStringUnion(Iterable)
+ * @see Automata#makeBinaryStringUnion(Iterable)
  * @see Automata#makeStringUnion(BytesRefIterator)
  * @see Automata#makeBinaryStringUnion(BytesRefIterator)
  */
@@ -238,7 +237,7 @@ private Automaton completeAndConvert() {
    * UTF-8 codepoints as transition labels or binary (compiled) transition labels based on {@code
    * asBinary}.
    */
-  static Automaton build(Collection<BytesRef> input, boolean asBinary) {
+  static Automaton build(Iterable<BytesRef> input, boolean asBinary) {
     final StringsToAutomaton builder = new StringsToAutomaton();
 
     for (BytesRef b : input) {
@@ -273,9 +272,11 @@ private void add(BytesRef current, boolean asBinary) {
               + current);
     }
     assert stateRegistry != null : "Automaton already built.";
-    assert previous == null || previous.get().compareTo(current) <= 0
-        : "Input must be in sorted UTF-8 order: " + previous.get() + " >= " + current;
-    assert setPrevious(current);
+    if (previous != null && previous.get().compareTo(current) > 0) {
+      throw new IllegalArgumentException(
+          "Input must be in sorted UTF-8 order: " + previous.get() + " >= " + current);
+    }
+    setPrevious(current);
 
     // Reusable codepoint information if we're building a non-binary based automaton
     UnicodeUtil.UTF8CodePoint codePoint = null;