diff --git a/cwms-data-api/src/main/java/cwms/cda/api/LevelsController.java b/cwms-data-api/src/main/java/cwms/cda/api/LevelsController.java
index ff23602077..d681878e4f 100644
--- a/cwms-data-api/src/main/java/cwms/cda/api/LevelsController.java
+++ b/cwms-data-api/src/main/java/cwms/cda/api/LevelsController.java
@@ -265,6 +265,9 @@ public void getAll(@NotNull Context ctx) {
 
             String office = ctx.queryParam(OFFICE);
             String unit = ctx.queryParamAsClass(UNIT, String.class).getOrDefault(UnitSystem.SI.getValue());
+            if (!unit.equalsIgnoreCase(UnitSystem.SI.getValue()) && !unit.equalsIgnoreCase(UnitSystem.EN.getValue())) {
+                throw new IllegalArgumentException(String.format("Provided unit system is not supported: %s", unit));
+            }
             String datum = ctx.queryParam(DATUM);
             String begin = ctx.queryParam(BEGIN);
             String end = ctx.queryParam(END);
diff --git a/cwms-data-api/src/test/java/cwms/cda/api/LevelsControllerTestIT.java b/cwms-data-api/src/test/java/cwms/cda/api/LevelsControllerTestIT.java
index a0ff351a24..40d1293695 100644
--- a/cwms-data-api/src/test/java/cwms/cda/api/LevelsControllerTestIT.java
+++ b/cwms-data-api/src/test/java/cwms/cda/api/LevelsControllerTestIT.java
@@ -1849,6 +1849,79 @@ enum GetAllTestLegacy {
         }
     }
 
+    @Test
+    void test_get_constants_over_time() throws Exception {
+        String locId = "level_get_constants_test";
+        String levelId = locId + ".Stor.Ave.1Day.Regulating";
+        createLocation(locId, true, OFFICE);
+        final ZonedDateTime time = ZonedDateTime.of(2023, 6, 1, 0, 0, 0, 0, ZoneId.of("America"
+            + "/Los_Angeles"));
+        CwmsDataApiSetupCallback.getDatabaseLink().connection(c -> {
+            LocationLevel level = new ConstantLocationLevel.Builder(levelId, time)
+                .withOfficeId(OFFICE)
+                .withConstantValue(1.0)
+                .withLevelUnitsId("ac-ft")
+                .build();
+            LocationLevel level2 = new ConstantLocationLevel.Builder(levelId, time.plusDays(1))
+                .withOfficeId(OFFICE)
+                .withConstantValue(2.0)
+                .withLevelUnitsId("ac-ft")
+                .build();
+            DSLContext dsl = dslContext(c, OFFICE);
+            LocationLevelsDaoImpl dao = new LocationLevelsDaoImpl(dsl);
+            dao.storeLocationLevel(level);
+            dao.storeLocationLevel(level2);
+        });
+
+        Instant startTime = Instant.parse("2023-06-01T00:00:00Z");
+        Instant endTime = Instant.parse("2023-06-02T12:00:00Z");
+
+        // get location level constants over time
+        given()
+            .log().ifValidationFails(LogDetail.ALL, true)
+            .accept(Formats.JSONV2)
+            .contentType(Formats.JSONV2)
+            .queryParam(Controllers.OFFICE, OFFICE)
+            .queryParam(START, startTime.toString())
+            .queryParam(END, endTime.toString())
+            .queryParam(LEVEL_ID_MASK, levelId)
+            .queryParam(UNIT, "EN")
+        .when()
+            .redirects().follow(true)
+            .redirects().max(3)
+            .get("/levels/")
+        .then()
+            .log().ifValidationFails(LogDetail.ALL, true)
+        .assertThat()
+            .statusCode(is(HttpServletResponse.SC_OK))
+            .body("levels.size()", is(2))
+            .body("levels[0].constant-value", equalTo(1.0f))
+            .body("levels[1].constant-value", equalTo(2.0f))
+        ;
+
+        // get location level constants over time
+        given()
+            .log().ifValidationFails(LogDetail.ALL, true)
+            .accept(Formats.JSONV2)
+            .contentType(Formats.JSONV2)
+            .queryParam(Controllers.OFFICE, OFFICE)
+            .queryParam(START, startTime.toString())
+            .queryParam(END, "2023-06-01T12:00:00Z")
+            .queryParam(LEVEL_ID_MASK, levelId)
+            .queryParam(UNIT, "EN")
+            .when()
+            .redirects().follow(true)
+            .redirects().max(3)
+            .get("/levels/")
+            .then()
+            .log().ifValidationFails(LogDetail.ALL, true)
+            .assertThat()
+            .statusCode(is(HttpServletResponse.SC_OK))
+            .body("levels.size()", is(1))
+            .body("levels[0].constant-value", equalTo(1.0f))
+        ;
+    }
+
     enum GetAllTestNewAliases {
         DEFAULT(Formats.DEFAULT, Formats.JSONV2),
         JSON(Formats.JSON, Formats.JSONV2),