diff --git a/query-algebrizer/src/clauses/where_fn.rs b/query-algebrizer/src/clauses/where_fn.rs
index 40c047fb..3dade69f 100644
--- a/query-algebrizer/src/clauses/where_fn.rs
+++ b/query-algebrizer/src/clauses/where_fn.rs
@@ -159,7 +159,12 @@ impl ConjoiningClauses {
                 match self.bound_value(&in_var) {
                     // The type is already known if it's a bound variable….
                     Some(ref in_value) => Ok(Val(in_value.clone())),
-                    None => bail!(ErrorKind::UnboundVariable((*in_var.0).clone())),
+                    None => {
+                        // The variable is present in `:in`, but it hasn't yet been provided.
+                        // This is a restriction we will eventually relax: we don't yet have a way
+                        // to collect variables as part of a computed table or substitution.
+                        bail!(ErrorKind::UnboundVariable((*in_var.0).clone()))
+                    },
                 }
             },
 
diff --git a/query-algebrizer/tests/ground.rs b/query-algebrizer/tests/ground.rs
index 0a2bdf69..b7f811db 100644
--- a/query-algebrizer/tests/ground.rs
+++ b/query-algebrizer/tests/ground.rs
@@ -13,6 +13,8 @@ extern crate mentat_query;
 extern crate mentat_query_algebrizer;
 extern crate mentat_query_parser;
 
+use std::collections::BTreeMap;
+
 use mentat_core::{
     Attribute,
     Entid,
@@ -37,7 +39,9 @@ use mentat_query_algebrizer::{
     ComputedTable,
     Error,
     ErrorKind,
+    QueryInputs,
     algebrize,
+    algebrize_with_inputs,
 };
 
 // These are helpers that tests use to build Schema instances.
@@ -92,6 +96,11 @@ fn bails(schema: &Schema, input: &str) -> Error {
     algebrize(schema.into(), parsed).expect_err("algebrize to have failed")
 }
 
+fn bails_with_inputs(schema: &Schema, input: &str, inputs: QueryInputs) -> Error {
+    let parsed = parse_find_string(input).expect("query input to have parsed");
+    algebrize_with_inputs(schema, parsed, 0, inputs).expect_err("algebrize to have failed")
+}
+
 fn alg(schema: &Schema, input: &str) -> ConjoiningClauses {
     let parsed = parse_find_string(input).expect("query input to have parsed");
     algebrize(schema.into(), parsed).expect("algebrizing to have succeeded").cc
@@ -313,3 +322,26 @@ fn test_ground_nonexistent_variable_invalid() {
         },
     }
 }
+
+#[test]
+fn test_unbound_input_variable_invalid() {
+    let schema = prepopulated_schema();
+    let q = r#"[:find ?y ?age :in ?x :where [(ground [?x]) [?y ...]] [?y :foo/age ?age]]"#;
+
+    // This fails even if we know the type: we don't support grounding bindings
+    // that aren't known at algebrizing time.
+    let mut types = BTreeMap::default();
+    types.insert(Variable::from_valid_name("?x"), ValueType::Ref);
+
+    let i = QueryInputs::new(types, BTreeMap::default()).expect("valid QueryInputs");
+
+    let e = bails_with_inputs(&schema, &q, i);
+    match e {
+        Error(ErrorKind::UnboundVariable(v), _) => {
+            assert_eq!(v.0, "?x");
+        },
+        _ => {
+            panic!();
+        },
+    }
+}