diff --git a/bigdata-core/bigdata-rdf/src/java/com/bigdata/rdf/sparql/ast/StaticAnalysis.java b/bigdata-core/bigdata-rdf/src/java/com/bigdata/rdf/sparql/ast/StaticAnalysis.java
index 1f139fd5ae..ab6101bd88 100644
--- a/bigdata-core/bigdata-rdf/src/java/com/bigdata/rdf/sparql/ast/StaticAnalysis.java
+++ b/bigdata-core/bigdata-rdf/src/java/com/bigdata/rdf/sparql/ast/StaticAnalysis.java
@@ -2047,29 +2047,20 @@ public static INeedsMaterialization.Requirement gatherVarsToMaterialize(
                 
                 final INeedsMaterialization bop2 = (INeedsMaterialization) bop;
                                
-                final Set<IVariable<IV>> t = getVarsFromArguments(bop);
+                final Collection<IVariable<IV>> varsFromArguments = getVarsFromArguments(bop);
+                // Need to collect vars into a new set, as adding referenced vars into the original one (varsFromArguments)
+                // results in ConcurrentModificationException. The use of recursive method collectVarsFromExpressions also
+                // fixes references of derived vars, for example ?a+?b AS ?c, 1+?c AS ?d
+                // Ref: Test_Ticket_T172113
+                final Collection<IVariable<IV>> t = new LinkedHashSet<>(varsFromArguments);
                 
                 // https://jira.blazegraph.com/browse/BLZG-2083 (str() produces NotMaterializedException when using group by/sample)
                 // NotMaterializedException can be thrown by a function referenced in group by, to avoid that all variables referenced
                 // in group by expressions should be materialized, as MemoryGroupByOp needs real IVs. To avoid excessive adding of
                 // unneeded variables, varMap is provided as a reference of variables which might need materialization.
                 if (varMap != null) {
-                    
-                    for (IVariable<IV> key : t) {
-                        
-                        IValueExpression expr = varMap.get(key);
-                        
-                        if (expr != null) {
-                            
-                            Set<IVariable<IV>> vars = getVarsFromArguments(expr);
-                            
-                            t.addAll(vars);
-                            
-                        }
-                        
-                     }
-                    
-                 }
+                    collectVarsFromExpressions(varMap, varsFromArguments, t);
+                }
                 
                 if (t.size() > 0) {
                     
@@ -2096,6 +2087,26 @@ public static INeedsMaterialization.Requirement gatherVarsToMaterialize(
     
     }
 
+    @SuppressWarnings({ "rawtypes" })
+    private static void collectVarsFromExpressions(Map<IVariable<?>, IValueExpression<?>> varMap,
+            Collection<IVariable<IV>> varsFromArguments, Collection<IVariable<IV>> result) {
+        for (IVariable<IV> key : varsFromArguments) {
+            
+            IValueExpression expr = varMap.get(key);
+            
+            if (expr != null) {
+                
+                Set<IVariable<IV>> vars = getVarsFromArguments(expr);
+
+                collectVarsFromExpressions(varMap, vars, result);
+
+                result.addAll(vars);
+                
+            }
+
+         }
+    }
+
     @SuppressWarnings({ "rawtypes", "unchecked" })
     private static Set<IVariable<IV>> getVarsFromArguments(final BOp c) {
     
diff --git a/bigdata-sails-test/src/test/java/com/bigdata/rdf/sail/webapp/Test_Ticket_T172113.java b/bigdata-sails-test/src/test/java/com/bigdata/rdf/sail/webapp/Test_Ticket_T172113.java
new file mode 100644
index 0000000000..e54c136485
--- /dev/null
+++ b/bigdata-sails-test/src/test/java/com/bigdata/rdf/sail/webapp/Test_Ticket_T172113.java
@@ -0,0 +1,74 @@
+/*
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; version 2 of the License.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program; if not, write to the Free Software
+Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+*/
+
+
+package com.bigdata.rdf.sail.webapp;
+
+import junit.framework.Test;
+
+import org.openrdf.query.QueryLanguage;
+import org.openrdf.query.TupleQuery;
+import org.openrdf.query.TupleQueryResult;
+
+import com.bigdata.journal.IIndexManager;
+import com.bigdata.rdf.sail.webapp.client.RemoteRepository.RemoveOp;
+
+/**
+ * ConcurrentModificationException on non-grouping query with aggregates in SELECT.
+ * @See https://phabricator.wikimedia.org/T172113
+ */
+public class Test_Ticket_T172113<S extends IIndexManager> extends
+		AbstractTestNanoSparqlClient<S> {
+
+	public Test_Ticket_T172113() {
+
+	}
+
+	public Test_Ticket_T172113(final String name) {
+
+		super(name);
+
+	}
+
+	public static Test suite() {
+
+		return ProxySuiteHelper.suiteWhenStandalone(Test_Ticket_T172113.class,
+                "test.*", TestMode.triples
+                );
+       
+	}
+
+   /**
+    * Test supposed to check if constants .. will be resolved
+    */
+   public void test_aggregatesWithoutGroupBy_T172113() throws Exception {
+        // Clear DB per task description (Executing the query over the empty database)
+        m_repo.remove(new RemoveOp(null, null, null));
+        String query = "SELECT (COUNT(*) AS ?a) (COUNT(?x) AS ?b) (?b/?a AS ?r) { BIND(1 AS ?x)}";
+        final TupleQuery tq = m_repo.getBigdataSailRemoteRepository().getConnection().prepareTupleQuery(QueryLanguage.SPARQL, query, null);
+        final TupleQueryResult tqr = tq.evaluate();
+        try {
+            int count = 0;
+            while (tqr.hasNext()) {
+                System.out.println(tqr.next());
+                count++;
+             }
+            assertEquals(1,count); // asserting successful execution of the query, as it was failing while parsing
+        } finally {
+            tqr.close();
+        }
+   }
+
+}