diff --git a/icu4c/source/test/intltest/dtptngts.cpp b/icu4c/source/test/intltest/dtptngts.cpp
index 4a55bb719e3..3fdafe40fcb 100644
--- a/icu4c/source/test/intltest/dtptngts.cpp
+++ b/icu4c/source/test/intltest/dtptngts.cpp
@@ -1416,6 +1416,11 @@ void IntlTestDateTimePatternGeneratorAPI::test20640_HourCyclArsEnNH() {
         {"ars", u"h a", u"h:mm a"},
         // en_NH is interesting because NH is a depregated region code.
         {"en_NH", u"h a", u"h:mm a"},
+        // ch_ZH is a typo (should be zh_CN), but we should fail gracefully.
+        // {"cn_ZH", u"HH", u"H:mm"}, // TODO(ICU-20653): Desired behavior
+        {"cn_ZH", u"HH", u"h:mm a"}, // Actual behavior
+        // a non-BCP47 locale without a country code should not fail
+        {"ja_TRADITIONAL", u"H時", u"H:mm"},
     };
 
     for (auto& cas : cases) {
diff --git a/icu4j/main/classes/core/src/com/ibm/icu/text/DateTimePatternGenerator.java b/icu4j/main/classes/core/src/com/ibm/icu/text/DateTimePatternGenerator.java
index 81e2fce492b..a730503f1bb 100644
--- a/icu4j/main/classes/core/src/com/ibm/icu/text/DateTimePatternGenerator.java
+++ b/icu4j/main/classes/core/src/com/ibm/icu/text/DateTimePatternGenerator.java
@@ -351,6 +351,8 @@ public class DateTimePatternGenerator implements Freezable<DateTimePatternGenera
         String language = uLocale.getLanguage();
         String country = uLocale.getCountry();
         if (language.isEmpty() || country.isEmpty()) {
+            // Note: addLikelySubtags is documented not to throw in Java,
+            // unlike in C++.
             ULocale max = ULocale.addLikelySubtags(uLocale);
             language = max.getLanguage();
             country = max.getCountry();
@@ -368,9 +370,13 @@ public class DateTimePatternGenerator implements Freezable<DateTimePatternGenera
 
         // Check if the region has an alias
         if (list == null) {
-            Region region = Region.getInstance(country);
-            country = region.toString();
-            list = getAllowedHourFormatsLangCountry(language, country);
+            try {
+                Region region = Region.getInstance(country);
+                country = region.toString();
+                list = getAllowedHourFormatsLangCountry(language, country);
+            } catch (IllegalArgumentException e) {
+                // invalid region; fall through
+            }
         }
 
         if (list != null) {
diff --git a/icu4j/main/tests/core/src/com/ibm/icu/dev/test/format/DateTimeGeneratorTest.java b/icu4j/main/tests/core/src/com/ibm/icu/dev/test/format/DateTimeGeneratorTest.java
index b5874dc02a3..767bb60ef26 100644
--- a/icu4j/main/tests/core/src/com/ibm/icu/dev/test/format/DateTimeGeneratorTest.java
+++ b/icu4j/main/tests/core/src/com/ibm/icu/dev/test/format/DateTimeGeneratorTest.java
@@ -1736,6 +1736,11 @@ public class DateTimeGeneratorTest extends TestFmwk {
             {"ars", "h a", "h:mm a"},
             // en_NH is interesting because NH is a depregated region code.
             {"en_NH", "h a", "h:mm a"},
+            // ch_ZH is a typo (should be zh_CN), but we should fail gracefully.
+            // {"cn_ZH", "HH", "H:mm"}, // TODO(ICU-20653): Desired behavior
+            {"cn_ZH", "HH", "h:mm a"}, // Actual behavior
+            // a non-BCP47 locale without a country code should not fail
+            {"ja_TRADITIONAL", "H時", "H:mm"},
         };
 
         for (String[] cas : cases) {